What P0499 means
The evaporative emission (EVAP) system stores fuel vapour from the tank in a charcoal canister; a purge valve lets the engine draw it off, and a vent valve opens the canister to fresh air.
How the engine computer detects it
The vent valve is normally open and switched to ground by the module (GM scheme P0449/P0498/P0499; Ford uses P0446). P0499 means the control wire is shorted to positive: with the driver on the output stays high – GM sees 0.5 Ω or less to the supply, Saab above 4.6 V; MIL after two trips. The valve then cannot close, the system cannot be sealed, and the leak tests are suspended because they would otherwise report a large leak.
Written from public technical sources and not yet checked by a mechanic. How we write these pages
Symptoms
- Check engine light on
- EVAP readiness not set
- Usually no change in driving
Common causes
- Control wire touching battery positive or the valve supply (chafed harness) Most likely
- Vent valve coil with shorted turns, driver switches off Possible
- Moisture or corrosion in the connector Possible
- Driver in the engine module Less likely
How to diagnose P0499
- Read every stored code with an OBD-II scanner. If misfire or fuel trim codes are there too, fix those first: they often cause this one.
- Inspect the parts involved: evap vent valve, vent valve wiring, engine control module.
- Work through the causes above from most to least likely, cheapest checks first.
- Clear the code and drive a full drive cycle to confirm the repair held.
Vehicles that report P0499 most
| Vehicle | Years | Reports | Share | Median miles |
|---|---|---|---|---|
| Chevrolet Silverado 1500 | 2019–2020 | 3 | — |
Counts of owner complaints to the NHTSA that mention P0499 (4 in total), not failure rates: popular models get more complaints simply because more of them are on the road. Models with fewer than 3 complaints are left out.
Reading the code
- P
- Powertrain
- 0
- Generic (SAE) code
- 4
- Auxiliary emission controls
- 99
- Specific fault
Questions drivers ask
Can I drive with code P0499?
Usually yes for a short time, if the car drives normally, the temperature gauge reads normal and the check engine light is steady. If the light flashes, the engine loses power or overheats, or other warning lights come on, pull over and have it checked.
Will my car pass an emissions inspection with P0499?
Probably not in states that test vehicles with an OBD-II scan: an emissions-related code that keeps the check engine light on is usually an automatic fail. Rules differ by state and county, so check yours.
Will P0499 clear by itself?
The light can go off after several drive cycles without the fault, but the code returns if the cause is still there. After a repair, the readiness monitors need a full drive cycle before an inspection.
Can I fix P0499 myself?
Often yes, with an OBD-II scanner and basic tools. Difficulty: easy. Start with the cheapest checks before replacing expensive parts.
This page is general information, not a substitute for a professional diagnosis. Repair costs are rough estimates for parts and labor and vary by vehicle and region.