P042E: EGR A Control Stuck Open
P042E means the exhaust gas recirculation (EGR) valve is more open than the engine computer commands and cannot close. It is moderately serious: the car may still run, but rough idle, stalling and loss of power are possible, so have it checked soon.
You can drive, but get it checked soon
The engine may run acceptably, but an EGR valve stuck open can cause stalling, rough running and reduced power, possibly limp mode (reduced power to protect the engine). Stalling in traffic is a safety risk, so limit driving to getting the car checked.
What it means
The EGR system sends part of the exhaust back into the intake to lower combustion temperature and reduce nitrogen oxides (NOx), a regulated pollutant. P042E is set when the position-controlled EGR valve "A" stays more open than commanded and the computer cannot close it. Too much recirculated exhaust upsets combustion, which can cause rough idle, stalling and smoke. When the fault is set, EGR control is switched off. The check engine light comes on, and on some GM diesels it lights after a single trip. Detection details differ by manufacturer: Ford petrol engines compare valve position with limits, while Ford diesels look for a position deviation that lasts for a set time.
Likely causes
- Valve or disc coked, does not closeVery likely
Carbon deposits build up on the valve and its seat and can hold the valve open.
- Actuator motor, gear or return spring faultyPossible
If the motor, gears or spring that closes the valve fail, the valve cannot return to the closed position.
- Position sensor in the valve reads wrongPossible
A wrong position signal makes the computer think the valve is open when it may not be.
- Weak supply to the drive or connector faultLess likely
Low voltage or a poor connection can keep the actuator from driving the valve closed.
What you may notice
- Check engine light on (GM after one trip)
- Rough idle, stalling, starting problems
- Loss of power, possibly limp mode
- Diesel: smoke, higher soot load
What to check, cheapest first
- 1Read the code and any related codes (such as P042F, P0404, P0405) with a scan tool, and note when they were set. · you can do it
- 2Look at the EGR valve connector and wiring for corrosion, looseness or damage. · you can do it
- 3Check the battery and charging voltage, since weak supply can affect the valve drive. · you can do it
- 4With a scan tool, compare commanded and actual EGR position while commanding the valve closed. · mechanic
- 5Remove the valve and inspect it for carbon buildup, a stuck disc, or a broken spring or gear. · mechanic
- 6Clean or replace the valve as needed, then clear the code and confirm it does not return. · mechanic
Related codes
Frequently asked questions
Can I drive with P042E?+
Short trips to a repair shop are usually possible, but the engine can stall, run rough or lose power. Avoid long drives and heavy traffic until it is fixed.
Can a loose gas cap cause P042E?+
No. A gas cap affects fuel vapor (evaporative) codes. P042E points to the EGR valve, its actuator, sensor or wiring.
Will P042E pass an emissions test?+
Unlikely. The code is emissions-relevant and turns the check engine light on, which generally results in a failed test until it is repaired.
Written with the help of AI from the public OBDex definition (CC0). Each make may use the code differently. Updated: 2026-11-10