P0400

What it means

P0400 is the generic SAE J2012 code for a fault in the exhaust gas recirculation (EGR) 'A' flow. The engine control unit commands the EGR system to open or close and then checks the result, usually through an EGR position sensor, a differential pressure sensor, or changes in manifold pressure and fuel trim. When the measured flow does not match what was commanded, or the circuit is faulty, the code is set. The exact monitoring strategy varies with the make, model and engine.

Symptoms

• Engine warning light on
• Rough or unstable idle
• Hesitation or poor pickup under light load
• Occasional stalling at idle or when coming to a stop
• Pinking or knocking under load
• Failed emissions test with higher NOx readings

Possible causes

• EGR valve sticking or clogged with carbon so it cannot open or close properlylikely
• Blocked EGR passages, pipe or cooler restricting flowlikely
• Faulty EGR valve actuator, motor or integral position sensorpossible
• Wiring or connector fault in the EGR valve or sensor circuit (open circuit, short, corrosion)possible
• Vacuum supply fault on vacuum-operated systems: split hose, leaking diaphragm or faulty vacuum solenoidpossible
• Faulty differential pressure or manifold pressure sensor or its blocked sensing pipes, where fittedpossible
• Engine control unit fault or outdated calibration where a manufacturer bulletin confirms itrare

How to diagnose

1. Read all stored and pending codes plus freeze frame data to see engine load, speed and temperature when the code set, and note any related codes
2. Visually inspect the EGR valve, pipes, vacuum hoses and wiring for damage, leaks, soot deposits and loose connectors
3. Check for other faults that upset the readings, such as intake or exhaust leaks and MAP or MAF sensor faults
4. Using live data, compare commanded and actual EGR position or flow, and watch manifold pressure and fuel trims when the valve is operated
5. Command the EGR valve with a bidirectional scan function and check it moves smoothly and the idle changes as expected, following the model-specific procedure
6. On vacuum-operated systems, test the vacuum supply, the solenoid and whether the valve holds vacuum
7. Check the supply, earth and signal wiring at the valve and ECU using the manufacturer's wiring data for that engine
8. Remove the valve and inspect the passages and cooler for blockage, where the engine design allows

How to fix

• Clean or replace the EGR valve if it is sticking or its actuator or position sensor has failed
• Clear carbon from blocked EGR passages and pipes, or replace the pipe or cooler if it is restricted or damaged
• Repair or replace damaged wiring and connectors in the EGR circuit
• Replace split vacuum hoses, a leaking diaphragm or a faulty vacuum solenoid
• Replace a faulty differential pressure sensor or clear its blocked sensing pipes
• Apply a manufacturer-approved ECU software update only if a bulletin identifies an outdated calibration as the cause

Can I keep driving?

Nearby codes

• P03FFGenericIgnition L Control Signal Circuit High
• P0401GenericEGR A Flow Insufficient Detected