Introduction
This page is a diagnostic procedure, not a problem catalog.
If you already know you need to troubleshoot your generator, start here. Follow the steps in order. Do not skip ahead. Each step tells you what to test, what the result means, and where to go next.
This page covers the complete diagnostic tree for 10 common failures. For deep-dives on specific symptoms, read the dedicated guides referenced by title at the end of each section.
Generator Troubleshooting Flowchart
This is the core diagnostic tree for the entire page. Every section below feeds back into these four questions.
text
┌─────────────────────────────────────────────┐
│ Step 1: Does the engine crank? │
└─────────────────────────────────────────────┘
│
┌───────┴───────┐
↓ ↓
NO YES
↓ ↓
┌───────────┐ ┌─────────────────────────────┐
│ Battery │ │ Step 2: Does it fire? │
│ Starter │ └─────────────────────────────┘
│ Pull cord │ │
│ Mech lock │ ┌───────┴───────┐
└───────────┘ ↓ ↓
NO YES
↓ ↓
┌───────────┐ ┌─────────────────┐
│ Fuel │ │ Step 3: Does it │
│ Spark │ │ stay running? │
│ Ignition │ └─────────────────┘
└───────────┘ │
┌───────┴───────┐
↓ ↓
NO YES
↓ ↓
┌───────────┐ ┌─────────────────┐
│ Oil │ │ Step 4: Does it │
│ Choke │ │ run smoothly? │
│ Fuel │ └─────────────────┘
│ Vent/CO │ │
│ Thermal │ ┌───────┴───────┐
└───────────┘ ↓ ↓
NO YES
↓ ↓
┌───────────┐ ┌─────────────────┐
│ Rough │ │ Step 5: Does it │
│ Surging │ │ produce stable │
│ Choke dep │ │ output? │
│ Black smoke│ └─────────────────┘
└───────────┘ │
┌───────┴───────┐
↓ ↓
NO YES
↓ ↓
┌───────────┐ ┌─────────────┐
│ Breaker │ │ Load-specific│
│ AVR │ │ problem │
│ Inverter │ └─────────────┘
│ Alternator│
│ RPM │
└───────────┘
How to use this flowchart:
- Answer Step 1. If NO → go to Step 1 section. If YES → go to Step 2.
- Answer Step 2. If NO → go to Step 2 section. If YES → go to Step 3.
- Answer Step 3. If NO → go to Step 3 section. If YES → go to Step 4.
- Answer Step 4. If NO → go to Step 4 section. If YES → go to Step 5.
- Answer Step 5. If NO → go to Step 5 section. If YES → load-specific problem.
Step 1 — Generator Won’t Start
If the engine does not crank at all, the problem is mechanical or electrical — not fuel.
Diagnostic question: Does the engine crank?
| Result | Next Step |
|---|---|
| No crank | Continue this section |
| Cranks | Go to Step 2 — Generator Cranks but Does Not Start |
Check Fuel Supply
Even if the engine does not crank, confirm fuel supply before assuming an electrical fault.
| Test | Result | Next Step |
|---|---|---|
| Fuel valve open? | No | Open valve. Retry. |
| Fuel valve open? | Yes | Check fuel in tank |
| Fuel in tank? | No | Add fresh fuel |
| Fuel in tank? | Yes | Smell test |
| Smell test | Varnish/paint thinner | Drain stale fuel, add fresh |
| Smell test | Normal | Go to Check Oil Level |
Check Oil Level
Low oil sensor kills spark and can prevent start on many generators.
| Test | Result | Next Step |
|---|---|---|
| Dipstick between ADD and FULL | Yes | Go to Check Choke |
| Dipstick below ADD | No | Add oil to FULL. Retry. |
| Dipstick above FULL | Overfilled | Drain to FULL. Retry. |
Check Choke
| Test | Result | Next Step |
|---|---|---|
| Choke set to FULL for cold start | Yes | Go to Check Spark |
| Choke not FULL | No | Set to FULL. Pull 3–5 times. |
| Still no start after 5 pulls | — | Go to Check Spark |
Check Spark
| Test | Result | Next Step |
|---|---|---|
| Remove plug, connect to boot, ground to block | Blue spark | Ignition OK. Go to Interpret the Result. |
| No spark or yellow spark | Weak/no spark | Replace plug. Retest. |
| Still no spark after new plug | — | Check kill switch. Check oil level. Check ignition coil. |
| Kill switch in OFF | — | Set to RUN. Retest. |
| Oil level low | — | Add oil. Retest. |
Interpret the Result
| What You Found | What It Means | Next Branch |
|---|---|---|
| No crank, battery low | Battery/starter issue | Charge/replace battery. If still no crank, check starter relay. |
| No crank, pull cord locked | Flooded or hydraulic lock | Remove plug, crank to clear. If still locked, engine seized. |
| Cranks but no fire, fuel OK, spark OK | Compression issue | Check valve lash. Check compression. |
| Cranks but no fire, fuel OK, no spark | Ignition circuit | Check kill switch, oil sensor, coil. |
| Cranks but no fire, no fuel | Fuel supply | Check valve, filter, pump, carburetor. |
If engine cranks → go to Step 2 — Generator Cranks but Does Not Start.
If engine starts and stays running → go to Step 4 — Generator Runs but Runs Poorly.
Step 2 — Generator Cranks but Does Not Start
If the engine cranks but does not fire, the problem is fuel or spark.
Diagnostic question: Does it fire?
| Result | Next Step |
|---|---|
| No fire | Continue this section |
| Fires but dies | Go to Step 3 — Generator Starts Then Dies |
| Fires and runs | Go to Step 4 — Generator Runs but Runs Poorly |
Fuel vs Spark Diagnosis
| Test | Result | Next Step |
|---|---|---|
| Fuel reaches carburetor | Yes | Check spark |
| Fuel absent | No | Fuel supply |
| Spark present | Yes | Fuel/compression |
| Spark absent | No | Ignition circuit |
| Plug wet | Yes | Flooding |
| Plug dry | Yes | Fuel delivery |
Check Fuel Supply
| Test | Result | Next Step |
|---|---|---|
| Fuel valve open? | No | Open valve. Retry. |
| Fuel valve open? | Yes | Check fuel in tank |
| Fuel in tank? | No | Add fresh fuel |
| Fuel in tank? | Yes | Smell test |
| Smell test | Varnish/paint thinner | Drain stale fuel, add fresh |
| Smell test | Normal | Check primer bulb |
| Primer bulb (if equipped) | Not firm | Press 3x. Retry. |
| Primer bulb | Firm | Go to Check Spark |
Check Spark
| Test | Result | Next Step |
|---|---|---|
| Remove plug, connect to boot, ground to block | Blue spark | Ignition OK. Go to Check Oil Level. |
| No spark or yellow spark | Weak/no spark | Replace plug. Retest. |
| Still no spark after new plug | — | Check kill switch. Check oil level. Check ignition coil. |
| Kill switch in OFF | — | Set to RUN. Retest. |
Check Oil Level
| Test | Result | Next Step |
|---|---|---|
| Dipstick between ADD and FULL | Yes | Go to Check Compression |
| Dipstick below ADD | No | Add oil to FULL. Retest. |
| Dipstick above FULL | Overfilled | Drain to FULL. Retest. |
Check Compression
| Test | Result | Next Step |
|---|---|---|
| Pull cord feels normal | Yes | Compression OK. Go to Interpret the Result. |
| Pull cord feels too easy | No | Low compression. Check valve lash. Check rings. |
| Pull cord locked | — | Flooded or hydraulic lock. Remove plug, crank to clear. |
Interpret the Result
| What You Found | What It Means | Next Branch |
|---|---|---|
| Fuel OK, spark OK, compression OK | Timing or ignition timing | Check flywheel key. Check ignition timing. |
| Fuel OK, spark OK, compression low | Valve or rings | Adjust valve lash. Check compression again. |
| Fuel OK, no spark | Ignition circuit | Check kill switch, oil sensor, coil. |
| No fuel, spark OK | Fuel delivery | Check valve, filter, pump, carburetor. |
| Plug wet | Flooded | Dry plug, crank to clear. |
| Plug dry | No fuel delivery | Check fuel supply. |
If engine fires but dies → go to Step 3 — Generator Starts Then Dies.
If engine fires and runs → go to Step 4 — Generator Runs but Runs Poorly.
Step 3 — Generator Starts Then Dies
If the engine fires but dies, the problem is oil, choke, fuel, vent, CO, or thermal.
Diagnostic question: Does it stay running?
| Result | Next Step |
|---|---|
| Dies in seconds | Continue this section |
| Dies after 1–5 minutes | Continue this section |
| Dies after 10–30 minutes | Continue this section |
| Dies only under load | Continue this section |
| Won’t restart until cool | Continue this section |
| Stays running | Go to Step 4 — Generator Runs but Runs Poorly |
Shutdown Timing Diagnosis
| Shutdown Timing | Most Likely Cause | Next Step |
|---|---|---|
| Shuts down in seconds | Oil/safety sensor | Check oil level |
| Shuts down after 1–5 minutes | Fuel/choke | Check choke, fuel flow |
| Shuts down after 10–30 minutes | Vent/heat/CO | Check ventilation, CO sensor |
| Shuts down only under load | Load/governor/fuel | Check load, governor |
| Won’t restart until cool | Thermal ignition | Test coil hot vs cold |
Check Oil Level
| Test | Result | Next Step |
|---|---|---|
| Dipstick between ADD and FULL | Yes | Go to Check Choke |
| Dipstick below ADD | No | Add oil to FULL. Retest. |
| Dipstick above FULL | Overfilled | Drain to FULL. Retest. |
| Generator on slope | — | Move to level ground. Retest. |
Check Choke
| Test | Result | Next Step |
|---|---|---|
| Choke left on 30–60 seconds | Yes | Go to Check Fuel Flow |
| Choke opened too early | No | Leave choke on longer. Retest. |
| Starts on choke only, dies when choke opens | — | Pilot jet blocked. Clean pilot jet. |
Check Fuel Flow
| Test | Result | Next Step |
|---|---|---|
| Fuel valve fully open? | No | Open fully. Retest. |
| Fuel valve fully open? | Yes | Smell test |
| Smell test | Varnish/paint thinner | Drain stale fuel, add fresh |
| Smell test | Normal | Check pilot jet |
| Pilot jet blocked | — | Clean with 0.008″ wire and carb cleaner |
Check Vent/Heat/CO
| Test | Result | Next Step |
|---|---|---|
| Generator in enclosed space? | Yes | Move to open air. Retest. |
| CO sensor triggered? | Yes | Ventilate. Reset. Retest. |
| Overheating? | Yes | Let cool. Check cooling fins. |
Interpret the Result
| What You Found | What It Means | Next Branch |
|---|---|---|
| Dies in seconds, oil low | Low oil sensor | Add oil. If still dies, sensor fault. |
| Dies after 1–5 min, choke OK, fuel OK | Pilot jet blocked | Clean pilot jet. |
| Dies after 10–30 min, enclosed space | CO/vent | Move to open air. |
| Dies under load | Overload or governor | Reduce load. Check governor. |
| Won’t restart until cool | Thermal ignition | Test coil hot vs cold. Replace if failing. |
For detailed “starts then dies” diagnosis, read the dedicated guide titled:
Generator Starts But Shuts Off: Field Failure Troubleshooting Report
This page only provides the general diagnostic entry point. The dedicated guide covers the full field failure analysis.
If engine stays running → go to Step 4 — Generator Runs but Runs Poorly.
Step 4 — Generator Runs but Runs Poorly
If the engine runs but runs poorly, diagnose by symptom.
Diagnostic question: Does it run smoothly?
| Result | Next Step |
|---|---|
| Runs rough | Continue this section |
| Surging/hunting | Continue this section |
| Runs only with choke | Continue this section |
| Black smoke | Continue this section |
| Runs smoothly | Go to Step 5 — Generator Runs but Has No/Low Output |
Symptom → Meaning
| Symptom | Meaning | Next Step |
|---|---|---|
| RPM rises/falls repeatedly | Surging/hunting | Clean carburetor, check governor |
| Engine shakes/misfires | Rough running | Check plug, valve lash |
| Runs only with choke | Lean condition | Clean pilot jet |
| Black smoke | Rich condition | Check choke, air filter |
Surging/Hunting
| Test | Result | Next Step |
|---|---|---|
| Carburetor partially blocked? | Yes | Clean pilot jet and main jet |
| Governor linkage stuck? | Yes | Free linkage. Retest. |
| Still surging after cleaning? | — | Check governor spring. Check intake leak. |
For detailed surging diagnosis, read the dedicated guide in our generator troubleshooting series.
Rough Running
| Test | Result | Next Step |
|---|---|---|
| Spark plug fouled or wrong gap? | Yes | Clean or replace plug. Regap to 0.025″. |
| Valve lash tight? | Yes | Adjust intake 0.006″, exhaust 0.008″ |
| Still rough after plug and valve? | — | Check compression. Check ignition timing. |
For detailed rough running diagnosis, read the dedicated guide in our generator troubleshooting series.
Runs Only With Choke
| Test | Result | Next Step |
|---|---|---|
| Pilot jet blocked? | Yes | Clean with 0.008″ wire and carb cleaner |
| Intake leak? | Yes | Check gaskets, carburetor boot |
| Still choke-dependent after cleaning? | — | Check fuel mixture screw. Check main jet. |
For detailed choke-dependent diagnosis, read the dedicated guide in our generator troubleshooting series.
Black Smoke
| Test | Result | Next Step |
|---|---|---|
| Choke stuck on? | Yes | Free choke. Retest. |
| Air filter clogged? | Yes | Clean or replace air filter |
| Still black smoke? | — | Check carburetor float level. Check main jet. |
For detailed black smoke diagnosis, read the dedicated guide in our generator troubleshooting series.
Interpret the Result
| What You Found | What It Means | Next Branch |
|---|---|---|
| Surging, carburetor partially blocked | Lean condition | Clean carburetor |
| Rough running, plug fouled | Ignition or rich condition | Replace plug. Check choke. |
| Runs only with choke | Lean condition | Clean pilot jet |
| Black smoke | Rich condition | Check choke, air filter |
If engine runs smoothly → go to Step 5 — Generator Runs but Has No/Low Output.
Step 5 — Generator Runs but Has No/Low Output
If the engine runs but has no or low output, diagnose the electrical side.
Diagnostic question: Does it produce stable output?
| Result | Next Step |
|---|---|
| No output | Continue this section |
| Low output | Continue this section |
| Unstable output | Continue this section |
| Stable output | Load-specific problem |
Output Diagnosis Flow
text
Engine speed normal? ↓ Breaker? ↓ Output voltage? ↓ AVR/inverter? ↓ Brushes/capacitor/alternator?
Check Engine Speed
| Test | Result | Next Step |
|---|---|---|
| RPM normal? | Yes | Go to Check Breaker |
| RPM too low | No | Check governor. Check throttle. |
| RPM too high | No | Check governor. Check linkage. |
Check Breaker
| Test | Result | Next Step |
|---|---|---|
| Breaker tripped? | Yes | Reset breaker. Retest. |
| Breaker OK? | Yes | Go to Check Output Voltage |
Check Output Voltage
| Test | Result | Next Step |
|---|---|---|
| No voltage | — | Go to Check AVR/Inverter |
| Low voltage | — | Go to Check AVR/Inverter |
| Unstable voltage | — | Go to Check AVR/Inverter |
| Normal voltage | — | Load-specific problem |
Check AVR/Inverter
| Test | Result | Next Step |
|---|---|---|
| AVR failure? | Yes | Replace AVR |
| Inverter failure? | Yes | Replace inverter |
| AVR/inverter OK? | Yes | Go to Check Brushes/Capacitor/Alternator |
Check Brushes/Capacitor/Alternator
| Test | Result | Next Step |
|---|---|---|
| Brushes worn? | Yes | Replace brushes |
| Capacitor failed? | Yes | Replace capacitor |
| Alternator failed? | Yes | Replace alternator |
| All OK? | Yes | Check wiring. Check control board. |
For detailed no-output diagnosis, read the dedicated guides in our generator troubleshooting series.
If output is stable → load-specific problem.
Step 6 — Diagnose by Symptom
If you already know your symptom, jump to the corresponding step above. Do not skip steps. A simpler cause in an earlier step may be the real problem.
| Symptom | Go to |
|---|---|
| Won’t start | Step 1 & Step 2 |
| Cranks but won’t start | Step 2 |
| Starts then dies | Step 3 |
| Runs rough | Step 4 |
| RPM keeps changing | Step 4 |
| Runs only with choke | Step 4 |
| Black smoke | Step 4 |
| No AC output | Step 5 |
| Low voltage | Step 5 |
| Overload | Step 5 |
Fuel vs Spark Diagnosis
| Test | Result | Next Step |
|---|---|---|
| Fuel reaches carburetor | Yes | Check spark |
| Fuel absent | No | Fuel supply |
| Spark present | Yes | Fuel/compression |
| Spark absent | No | Ignition circuit |
| Plug wet | Yes | Flooding |
| Plug dry | Yes | Fuel delivery |
Rule: If it fires with starting fluid but dies → fuel system. If no spark at plug → ignition circuit.
Engine vs Electrical Diagnosis
| Symptom | Engine or Electrical? |
|---|---|
| Cranks but no fire | Engine (fuel/spark) |
| Fires but dies | Engine (oil/choke/fuel) |
| Runs but no output | Electrical (AVR/inverter/alternator) |
| Runs but low voltage | Electrical or RPM |
| Clicks but no crank | Electrical (battery/starter) |
Generator Troubleshooting Tools
Basic
| Tool | Purpose | When to Use |
|---|---|---|
| Spark plug socket (13/16″ or 5/8″) | Remove plug | Step 1, Step 2 |
| Screwdriver | General | All steps |
| Feeler gauge | Check spark plug gap | Step 2, Step 4 |
| Fuel container | Drain old fuel | Step 1, Step 2, Step 3 |
Electrical
| Tool | Purpose | When to Use |
|---|---|---|
| Multimeter | Test spark, continuity, voltage | Step 2, Step 5 |
| Spark tester | Test ignition | Step 2 |
| Clamp meter | Test output current | Step 5 |
Advanced
| Tool | Purpose | When to Use |
|---|---|---|
| Tachometer | Check RPM | Step 4, Step 5 |
| Compression tester | Check engine compression | Step 2 |
| Load tester | Test output under load | Step 5 |
When to use what:
- No spark → spark tester or multimeter.
- No output → multimeter or clamp meter.
- Rough running → feeler gauge, compression tester.
- Surging → tachometer.
The Correct Troubleshooting Order
Do not skip steps. Follow this order:
text
Visual inspection ↓ Fuel ↓ Oil/safety ↓ Spark ↓ Air/choke ↓ Compression ↓ Load ↓ Output ↓ Electrical/control
Why order matters:
- Fuel valve closed mimics no spark.
- Low oil mimics fuel starvation.
- Choke wrong mimics carburetor failure.
- Checking spark before fuel wastes time.
Common Diagnostic Mistakes
| Mistake | Consequence | Prevention |
|---|---|---|
| “Fuel looks fine” | Stale fuel can look clear | Smell test |
| “Slope is fine” | Low oil sensor triggers | Level ground required |
| “Choke is fine” | Wrong position for temperature | Full choke cold |
| “Oil looks fine” | Low oil sensor kills spark | Check dipstick every use |
| Skipping fuel valve | Most common no-start cause | Check first |
| Replacing carburetor before cleaning | Unnecessary cost | Clean pilot jet first |
Repair Cost
These are field estimates, not universal repair prices.
| Problem | Typical DIY | When Professional Help Is Needed |
|---|---|---|
| Stale fuel | Easy | Carb damage |
| Spark plug | Easy | No ignition |
| Low oil | Easy | Sensor fault |
| Carburetor restriction | Moderate | Internal damage |
| No compression | Advanced | Engine repair |
| AVR/control board | Advanced | Electrical diagnosis |
Rule: 80% of generator troubleshooting fixes cost $0. Do the simple checks before calling a technician.
When to Stop DIY
Stop and call a technician if:
- Engine seized (pull cord won’t move with plug out)
- No compression (pull cord feels too easy)
- AVR or control board failure
- Internal carburetor damage
- Electrical diagnosis beyond multimeter
Fix yourself if:
- Issue is stale fuel, old spark plug, low oil
- Generator under 8 years old
- Engine has compression (pull cord feels normal)
- Parts available (Honda, Yamaha, Champion, Generac)
FAQ
How do you troubleshoot a generator step by step?
Follow the diagnostic tree. Step 1: Does the engine crank? If no, check battery, starter, pull cord, mechanical lock. If yes, go to Step 2. Step 2: Does it fire? If no, check fuel and spark. If yes, go to Step 3. Step 3: Does it stay running? If no, check oil, choke, fuel, vent, CO, thermal. If yes, go to Step 4. Step 4: Does it run smoothly? If no, diagnose by symptom. If yes, go to Step 5. Step 5: Does it produce stable output? If no, check breaker, AVR, inverter, alternator, RPM. If yes, load-specific problem.
What should I check first when troubleshooting a generator?
Check the fuel valve first. 50% of no-starts are a closed fuel valve. Open it (lever parallel to fuel line). Then check oil level, fuel freshness (smell test), and choke position. These four checks fix 80% of no-starts.
What is the correct generator diagnostic procedure?
Visual inspection → fuel → oil/safety → spark → air/choke → compression → load → output → electrical/control. Do not skip steps. Fuel valve closed mimics no spark. Low oil mimics fuel starvation. Choke wrong mimics carburetor failure.
Do you have a generator troubleshooting checklist?
Yes. Step 1: Does it crank? Step 2: Does it fire? Step 3: Does it stay running? Step 4: Does it run smoothly? Step 5: Does it produce stable output? At each step, check the most likely cause first: fuel valve, oil level, fuel freshness, choke position, spark, compression.
How do I tell if a generator problem is fuel or ignition related?
Remove the spark plug. Connect it to the boot and ground it to the engine block. Pull the cord. Blue spark = ignition OK, problem is fuel. No spark or yellow spark = ignition problem. Also check if plug is wet (flooding) or dry (fuel delivery).
How do I troubleshoot a generator that cranks but won’t start?
Check fuel valve open. Check fuel fresh (smell test). Check oil level. Check spark (blue spark). Remove plug — is it wet? Flooded. Dry plug, crank to clear. These five checks cover 90% of cranks-but-won’t-start.
How do I troubleshoot a generator that starts then dies?
Check oil level first (35% of cases). Low oil sensor kills engine after 2–5 seconds. Add oil to FULL mark. Then check choke position (leave on 30–60 seconds). Then check fuel freshness. If starts on choke but dies when choke opens, clean pilot jet.
How do I troubleshoot a generator with no spark?
Check kill switch in RUN position. Check oil level (low oil kills spark). Replace spark plug if black or wet. Test spark with inline tester. Still no spark? Check ignition coil or magneto.
How do I troubleshoot a generator that runs but has no power?
Check engine speed (RPM). Check breaker. Check output voltage with multimeter. No voltage → AVR/inverter/alternator. Low voltage → RPM or AVR. Unstable voltage → governor or AVR. Check brushes, capacitor, alternator.
What tools do I need to troubleshoot a generator?
Basic: spark plug socket, screwdriver, feeler gauge, fuel container. Electrical: multimeter, spark tester, clamp meter. Advanced: tachometer, compression tester, load tester. Most fixes need no tools.
What order should I test generator components?
Visual inspection → fuel → oil/safety → spark → air/choke → compression → load → output → electrical/control. Do not skip steps. Fuel valve closed mimics no spark. Low oil mimics fuel starvation.
When should I stop troubleshooting and call a technician?
Stop if engine seized (pull cord won’t move with plug out), no compression (pull cord feels too easy), AVR or control board failure, internal carburetor damage, or electrical diagnosis beyond multimeter. Fix yourself if issue is stale fuel, old spark plug, low oil, and generator under 8 years old.
Related Guides
- Generator Starts But Shuts Off: Field Failure Troubleshooting Report
- Carburetor Jet Cleaning Guide
- Generator No AC Output Diagnosis
- Generator Runs But No Power Diagnosis
- Generator Low Voltage Diagnosis
- Maintenance Checklist for Monthly Generator Exercise