Is It Safe to Drive With a P0400 Code?
Common Symptoms of a P0400 Fault
EML on
Rough idle
Loss of power
Increased emissions
Possible diesel knock on diesel engines
Possible Causes of P0400
A P0400 code can be triggered by faults in several different vehicle systems. Identifying the specific root cause requires proper diagnostic equipment and live data analysis — not just reading the code itself.
EGR Valve Blockage / Failure
The EGR valve is blocked with carbon deposits or has failed mechanically.
EGR Pipe Blockage
Carbon deposits blocking the EGR passage or pipe.
EGR Valve Wiring Fault
Electrical fault in the EGR valve control circuit.
EGR Sensor Fault
A sensor monitoring EGR flow is giving incorrect readings.
Why we don't list the exact part: A P0400 code tells you what's happening but not why. Replacing parts based on the code alone without proper diagnosis is the most common way drivers waste money. A proper diagnosis identifies the exact cause first.
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P0400 and Your MOT
A stored P0400 fault code will cause your car to fail its MOT. The engine management light will be illuminated during the test, which is an automatic failure under current MOT testing standards. Even if the light clears temporarily, the code remains in the ECU memory and can be read by the tester's equipment.
Get the fault properly diagnosed and repaired first — then have the code cleared and confirm the light doesn't return before booking your test.