EGR valve sticking or clogged with carbon, so it cannot open or close fully; Blocked EGR passages, pipe or cooler restricting flow; Faulty EGR valve position sensor or internal actuator fault; Damaged wiring, poor connector contact or corrosion in the EGR valve circuit; Vacuum supply leak or faulty vacuum solenoid or hose on vacuum-operated systems; Faulty differential pressure, MAP or related sensor giving a wrong flow reading; Fault within the engine control unit or outdated calibration where a manufacturer bulletin identifies it.

P0404

EGR A Control Circuit Range/Performance.

Generic
Powertrain
Generic code
Medium severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
4: AreaAuxiliary emission controls
04: FaultThe specific fault in that area

What it means

P0404 means the engine control unit has found that the exhaust gas recirculation (EGR) system is not behaving as commanded. The actual EGR valve position or flow does not match what was requested, or the control circuit is outside its expected range. The exact test used varies by vehicle, for example valve position feedback, manifold pressure change or fuelling response.

Symptoms

Engine warning light on
Rough or unstable idle
Hesitation or poor pickup under light load
Slight increase in fuel consumption
Pinking or knocking under load on some engines
Stalling or surging at idle in some cases

Possible causes

EGR valve sticking or clogged with carbon, so it cannot open or close fullylikely
Blocked EGR passages, pipe or cooler restricting flowlikely
Faulty EGR valve position sensor or internal actuator faultpossible
Damaged wiring, poor connector contact or corrosion in the EGR valve circuitpossible
Vacuum supply leak or faulty vacuum solenoid or hose on vacuum-operated systemspossible
Faulty differential pressure, MAP or related sensor giving a wrong flow readingrare
Fault within the engine control unit or outdated calibration where a manufacturer bulletin identifies itrare

How to diagnose

Read all stored and pending codes and freeze frame data to see engine load, speed and temperature when the fault set, and look for related codes
Visually inspect the EGR valve wiring, connector and vacuum hoses for damage, corrosion, looseness or splits
Check for exhaust or intake leaks around the EGR valve, pipes and gaskets
Using a scan tool capable of live data, compare commanded and actual EGR position and watch whether the valve responds when commanded
Check the supply voltage, earth and signal wiring at the EGR connector against the manufacturer's data for that model
On vacuum-operated valves, test vacuum supply and the control solenoid, and check the valve holds vacuum
Remove the valve and inspect it and the passages for carbon build-up or a valve stuck open or closed, following the model's procedure
Check related sensors such as MAP or differential pressure sensors for plausible readings

How to fix

Clean carbon deposits from the EGR valve and passages where the valve is serviceable, or replace the valve if it is worn or sticking
Clear or replace blocked EGR pipes or a restricted cooler
Repair damaged wiring or connectors, or replace the position sensor or actuator if proven faulty
Replace split or perished vacuum hoses and faulty vacuum solenoids
Replace any faulty MAP or pressure sensor that is giving false readings
Where a manufacturer bulletin confirms it, update the engine control unit calibration

Can I keep driving?

Medium severity

The car can usually be driven for a short time, but book it in soon. A valve stuck open can cause poor running or stalling, and carbon build-up can get worse and cost more to fix if left.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.