What P0402 means
Exhaust gas recirculation (EGR) routes part of the exhaust back into the intake to lower combustion temperature and NOx.
How the engine computer detects it
P0402 means too much exhaust is recirculated, or exhaust flows although the valve should be closed – CARB explicitly requires a leaking valve to be detected. Ford's DPFE check at idle with EGR off expects no pressure difference above the engine-off value; Ford diesels compare the measured EGR rate with the expected one, GM diesels the fresh-air deviation in the opposite direction. Gasoline engines idle roughly or stall, diesels smoke and may lose power.
Written from public technical sources and not yet checked by a mechanic. How we write these pages
Symptoms
- Check engine light on
- Rough idle or stalling (gasoline)
- Jerking at part load
- Diesel: black smoke, loss of power
Common causes
- EGR valve stuck open, dirt under the seat Most likely
- Ford: DPFE sensor drifting or hose wrong Possible
- Diesel: air leak between air mass meter and turbo, MAF drift Possible
- Valve control opens unintentionally Less likely
How to diagnose P0402
- 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: egr valve, egr valve seat, dpfe sensor, maf sensor.
- 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.
Reading the code
- P
- Powertrain
- 0
- Generic (SAE) code
- 4
- Auxiliary emission controls
- 02
- Specific fault
Questions drivers ask
Can I drive with code P0402?
Drive gently and only as far as you need to. The engine computer may limit power to protect the engine, so avoid highways and heavy traffic until it's fixed. If the check engine light starts flashing, pull over.
Will my car pass an emissions inspection with P0402?
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 P0402 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 P0402 myself?
Often yes, with an OBD-II scanner and basic tools. Difficulty: medium. 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.