Generac Portable Generator Fault Codes and Warning Lights
Generac's portable generators report problems differently from its home standby units. Depending on the model, a problem shows up in one of three ways: a COsense badge that blinks red or yellow after an automatic shut-off; a Malfunction Indicator Lamp (MIL) on the GP15500EFI and GP18000EFI that flashes a two-digit code such as 6-1; or an engine that starts and shuts off right away, which Generac's troubleshooting guide traces to low oil level or a COsense shut-off. Find your model below, read the light, and follow Generac's correction.
| Symptom | Generac Portable Generator Fault Codes and Warning Lights |
|---|---|
| Applies to | Generac portable generators: GP6500, GP6500E and GP8000E (COsense lights); GP15500EFI and GP18000EFI (EFI blink codes) |
| Severity | Unit will not run |
| DIY | Check, then call — owner-level checks first, then a dealer |
| Last checked |
Carbon monoxide: if the COsense light is blinking red, the generator shut itself off because carbon monoxide was accumulating. Generac's manual says to move the generator to an open, outdoor area and point the exhaust away from people and occupied buildings before restarting it.
Key facts
| Question | Short answer | Source |
|---|---|---|
| Blinking red COsense light | The generator was shut off due to an accumulating CO hazard | Generac GP6500 / GP6500E / GP8000E manual |
| Blinking yellow COsense light | A COsense system fault; COsense no longer provides protection and the generator shut off | Generac GP6500 / GP6500E / GP8000E manual |
| How long the light blinks | At least five minutes | Generac GP6500 / GP6500E / GP8000E manual |
| EFI blink codes (GP15500EFI, GP18000EFI) | Long flashes of 1.2 seconds, then short flashes of 0.25 second; 6 long + 1 short = code 6-1 | Generac EFI manual and support article |
| Starts, then shuts off right away | Low oil level, or a COsense shut-off (red or yellow light) | Generac troubleshooting guide |
| Who to call | A Generac Independent Authorized Service Dealer (IASD) | Generac |
COsense red and yellow lights (GP6500, GP6500E, GP8000E)
COsense models have a COsense badge on the side of the generator. The GP6500 / GP6500E / GP8000E owner's manual describes both lights, and its troubleshooting table gives the cause and the correction for each one, in these words:
| Light | Cause | Correction |
|---|---|---|
| Blinking RED | COsense shut-off due to accumulating carbon monoxide if a red light blinks on the side panel badge. | Follow all safety instructions and relocate generator to an open area outside, far away from windows, doors and vents. |
| Blinking YELLOW | COsense shut-off due to a system fault if a yellow light blinks on the side panel badge. | Start to confirm yellow light blinks when/if generator shuts-off. If COsense continues to fault and shut-off, contact IASD. |
Red. The manual: "After a shut-off, a blinking RED light in the COsense badge on the side of the generator provides notification that the generator was shut off due to an accumulating CO hazard. The RED light will blink for at least five minutes after a CO shut-off. Move the generator to an open, outdoor area and point the exhaust away from people and occupied buildings. Once relocated to a safe area, the generator can be restarted and the proper electrical connections made to supply electrical power." It adds: "Introduce fresh air and ventilate the location where the generator had shut down."
Yellow. The manual: "If a COsense system fault has occurred and no longer provides protection, the portable generator is shut off automatically and the YELLOW light will blink for at least five minutes in the COsense badge to notify the user of the fault. The COsense module can only be diagnosed and repaired by a trained technician at the dealer. The generator can be re-started, but may continue to shut-off."
COsense also reacts to carbon monoxide from other fuel-burning sources nearby, such as another generator's exhaust pointed at it; the manual says such a shut-off "is not an error." It also says: "COsense is not a substitute for an indoor carbon monoxide alarm." Generac's general portable troubleshooting guide gives the same two corrections, in slightly different words, for units equipped with COsense, and the 15.5 / 18.0 kW EFI manual carries the same COsense description for units so equipped.
EFI blink codes (GP15500EFI and GP18000EFI)
Generac's fault-code article applies to the GP15500EFI and GP18000EFI, and the matching owner's manual (rated 15.5 / 18.0 kW) carries the same table. The Malfunction Indicator Lamp on the control panel "displays a 2-digit code by flashing a sequence of long and short flashes. Long flashes are 1.2 seconds, and short flashes are 0.25 second." Generac's example: "Fault code 6-1 is displayed by: 6 long blinks = 6, one second pause, 1 short blink = 1."
How the lamp behaves, per the manual:
- When the key is switched ON, the MIL lights for 0.5 second; that does not indicate a fault.
- With more than one fault, the MIL pauses two seconds between codes, then pauses five seconds and repeats the list.
- A shutdown fault stops the engine (or keeps it from starting). "Before the unit can be started, the key must be first turned to the OFF position to clear the active fault."
- The engine control module keeps faults for three ON-OFF key cycles. To see them, turn the key ON without starting the engine; after 30 seconds the MIL flashes the stored faults.
| Fault Code | Cause | Condition | Correction |
|---|---|---|---|
| 1-1 | CO Monitor | 1. Carbon Monoxide above limit. 2. Input voltage to module below normal range. | 1. Move generator outside. 2. Contact IASD. |
| 1-2 | Battery Voltage | 1. Battery voltage above normal operating range. | 1. Contact IASD. |
| 1-3 | Battery Voltage | 1. Battery voltage below normal operating range. | 1. Charge battery to correct level. |
| 5-2 | Engine Speed | 1. No or low fuel. 2. Engine speed below normal operating range. | 1. Fill fuel tank. 2. Contact IASD. |
| 6-1 | Engine Oil Pressure | 1. Low oil pressure. 2. Switch voltage not detected. | 1. Fill crankcase to correct level. 2. Contact IASD. |
| Other | Multiple | 1. Multiple | 1. Contact IASD. |
The manual's note under the table: "Contact an IASD for full list of fault codes." Generac's support article prints the same codes and says its table "provides common fault codes and is for informational purposes only." It words the first 1-1 correction differently: "Never run a generator indoors or in a partially enclosed area. Move the generator."
Engine starts, then shuts off right away
Generac's portable troubleshooting guide lists three issues under "Engine starts and shuts off right away": low oil level, a COsense shut-off with a red light, and a COsense shut-off with a yellow light. For low oil level its solution is "Fill the crankcase to the correct level." The GP6500 / GP6500E / GP8000E manual explains the shutdown: "The engine is equipped with a low oil level sensor to shut down the engine automatically when the oil level drops below a specified level. The engine will not run until the oil has been filled to the proper level." That manual's own troubleshooting table lists low oil level under "Engine will not start; or starts and runs rough" and "Engine shuts down during operation", and its "Engine starts and shuts off right away" row lists only the two COsense shut-offs. For oil type and capacity by model, see our generator oil guide.
No code, but something's wrong
Generac's quick reference guide covers portables with no fault code at all. It applies to most portable generators, and features marked "(if equipped)" vary by unit. These are its full rows for four common problems, in Generac's words:
| Problem | Issue | Solution |
|---|---|---|
| Engine is running, but AC output is not available | Circuit Breaker is OPEN | Reset Circuit Breaker |
| Engine is running, but AC output is not available | Poor connection or defective cord set | Check and repair |
| Engine is running, but AC output is not available | The connected device is bad. | Connect another device that is in good condition. |
| Engine is running, but AC output is not available | GFCI receptacle is OPEN (if equipped) | Correct ground fault and press the reset button on GFCI |
| Engine is running, but AC output is not available | Fault in the generator | Contact IASD |
| Engine runs well at no-load, but bogs when load is applied | Short circuit in a connected load | Disconnect shorted electrical load. |
| Engine runs well at no-load, but bogs when load is applied | Generator is overloaded | Reduce load - See the owner's manual for generator limits. |
| Engine runs well at no-load, but bogs when load is applied | The engine speed is too slow. | Contact IASD |
| Engine runs well at no-load, but bogs when load is applied | Shorted generator circuit | Contact IASD |
| Engine runs well at no-load, but bogs when load is applied | Clogged carburetor | Clean carburetor |
| Engine runs well at no-load, but bogs when load is applied | Stale or old fuel | Replace with fresh fuel. |
| Engine runs well at no-load, but bogs when load is applied | Clogged spark arrestor | Clean spark arrestor screen |
| Engine will not start or starts and runs rough | Fuel shut-off is OFF. | Turn the fuel shut-off ON. |
| Engine will not start or starts and runs rough | Dirty air filter | Clean or replace air filter(s) |
| Engine will not start or starts and runs rough | Out of fuel | Fill fuel tank |
| Engine will not start or starts and runs rough | Stale fuel | Drain the fuel tank and fill it with fresh fuel. |
| Engine will not start or starts and runs rough | The spark plug wire is not connected to the spark plug wire | Connect wire to spark plug |
| Engine will not start or starts and runs rough | Water in fuel | Drain the fuel tank and fill it with fresh fuel. |
| Engine will not start or starts and runs rough | Over choking | Set choke to no choke position. |
| Engine will not start or starts and runs rough | Low oil Level | Fill the crankcase to the correct level |
| Engine will not start or starts and runs rough | Bad spark plug | Replace spark plug |
| Engine will not start or starts and runs rough | Clogged carburetor | Clean carburetor |
| Engine will not start or starts and runs rough | Excessive rich fuel mixture | Contact IASD |
| Engine will not start or starts and runs rough | Intake valve stuck open or closed. | Contact IASD |
| Engine will not start or starts and runs rough | Engine lost compression | Contact IASD |
| Pull cord keeps pulling loose , does not start the generator, and requires manually coiling | The recoil spring is damaged | Contact a local IASD. |
Not sure the load fits? Use our generator wattage calculator. Owner's manuals for these models: Generac manuals by model.
When to call a dealer
- The yellow COsense light keeps coming back. Generac's manual: "If COsense continues to fault and shut-off, contact IASD." The COsense module "can only be diagnosed and repaired by a trained technician at the dealer."
- Your EFI code's correction is "Contact IASD": 1-2, the second condition of 1-1, 5-2 and 6-1, and "Other".
- A troubleshooting row ends in "Contact IASD", such as "Fault in the generator" or "Engine lost compression".
Frequently asked questions
Why does my Generac GP6500 keep shutting off with a red light?
A blinking red COsense light means the generator "was shut off due to an accumulating CO hazard." Generac's manual says to move the generator to an open, outdoor area and point the exhaust away from people and occupied buildings; once it's relocated to a safe area, it can be restarted.
What does a blinking yellow light mean on a Generac portable?
On COsense models it means a COsense system fault: COsense "no longer provides protection," and the generator shut itself off. The manual says the generator "can be re-started, but may continue to shut-off," and if COsense continues to fault and shut off, to contact an IASD.
What is code 6-1 on a Generac GP18000EFI?
Engine Oil Pressure. Generac's table gives two conditions, "Low oil pressure." and "Switch voltage not detected.", with the corrections "Fill crankcase to correct level." and "Contact IASD."
How do I read a Generac EFI blink code?
Count the long flashes (1.2 seconds each), then, after the pause, the short flashes (0.25 second each). Generac's example: 6 long blinks, a one-second pause, then 1 short blink is code 6-1.
Are these the same as the Generac standby codes?
No. Generac's home standby generators use their own numeric code lists: see the Guardian / Evolution list or the Next Generation list.