What is the difference between P0404 and P0401?

P0401 is about gas: the control unit cannot detect enough exhaust being recirculated. P0404 is about the valve's own movement: the control unit commanded a position, the valve's internal position sensor reported a different one, and nothing is said about whether gas actually flowed. They share causes, chiefly carbon build-up, but the diagnosis differs. P0401 is confirmed by looking at flow effects, P0404 by comparing commanded against actual valve position on a diagnostic tool.

Will P0404 fail an MOT?

Yes. The engine management lamp it illuminates is a major defect on petrol cars first used on or after 1 July 2003 and diesels from 1 July 2008. There is an additional risk with this particular code: some cars respond to it by entering a reduced-power mode, and if the engine cannot be held at fast idle for the petrol gas test or revved out for the diesel smoke test, the emissions test cannot be completed, which is recorded as a major failure in its own right.

Can a P0404 code cause limp mode?

Yes, on many vehicles. If the control unit cannot verify where the recirculation valve is sitting, it cannot predict how much exhaust gas is entering the engine, and the safe response on a turbocharged engine in particular is to restrict boost and power until the fault is resolved. Not every car does this — plenty run entirely normally with the code stored — but where it happens the loss of performance is deliberate rather than a symptom of damage.

Do I need to replace the EGR valve for P0404?

Not always, but more often than for the flow codes. If carbon is binding the pintle, a thorough clean can restore normal movement. If the position sensor inside the valve has worn, cleaning achieves nothing because the mechanism was moving correctly all along — and on high-mileage engines both problems tend to arrive together. Have the commanded and actual positions compared first, and check the connector and wiring, since heat damage there mimics a failed valve exactly.

Powertrain code · standard

P0404EGR valve not moving to the position it was told to

Standard definition: EGR Range/Performance

Last updated 27 July 2026

Quick answer

This is a code about the valve's own mechanism rather than the gas going through it. The control unit told the valve to move to a particular position, the valve's built-in position sensor reported something else, and the two do not reconcile. Carbon binding the valve and a worn position sensor account for most of them.

How the car works this out

Electronically operated recirculation valves carry a position sensor inside them that reports back where the valve pintle actually is. The control unit continuously compares the position it commanded with the position reported. A discrepancy that persists — the valve failing to reach its commanded opening, failing to return fully closed when the engine is switched off, or reporting a position that changes erratically — sets this code. Crucially it says nothing about whether exhaust gas is flowing, only that the actuator is not behaving as instructed.

What you might notice

The lamp, frequently with no other sign at all
Reduced power or limp mode on cars that respond to this fault by restricting the engine
A rough idle if the valve has stuck part-open
On some cars an audible whirring or buzzing from the valve as the motor fights a jammed pintle

What causes it

Most likely first. A code names the check that failed, not the part that failed it, so more than one of these can produce the same number.

1Carbon deposits binding the pintle so the motor cannot drive it to the commanded positionCommon
How to check: Sweep the valve through its full range on a diagnostic tool and watch the reported position. A trace that sticks, jumps or refuses to reach the ends of travel is a mechanically bound valve.
2A worn position sensor track inside the valve, reporting nonsense even when the valve is moving correctlyCommon
How to check: The same sweep distinguishes it: a valve you can hear and feel moving smoothly while the reported position jumps around has a failing sensor rather than a jammed pintle.
3A failed electric motor in the valve assemblyPossible
4Corroded or chafed wiring and connectors, particularly where the loom runs near the exhaust manifold heatPossible
How to check: Wiggle the connector and loom with the valve being swept on a tool. A reading that flickers as you move the harness is a wiring fault, not a valve fault.
5A failed driver circuit in the control unit itselfLess common

What to do about it

1Have the commanded and actual positions compared on a diagnostic tool. That one test separates a jammed valve from a failed sensor from a wiring fault, and each of those is a different repair.
2Check the connector and loom before condemning the valve. Heat-hardened insulation and green corrosion in the plug are common on older cars and much cheaper to put right.
3If the valve is bound with carbon, cleaning may restore it, but be realistic — the position sensor track wears at the same time as the deposits build, so on a high-mileage engine a replacement valve is often the more durable answer.
4Fix it before the MOT rather than after. Cars that drop into a reduced-power mode with this code can be difficult to test at all.
5After replacement, some vehicles need the valve's closed position relearning with a diagnostic tool, so a code cleared without the relearn can return.

Can you keep driving?

Yes

The fault itself does not endanger anything, and many cars behave normally with it stored. The exception is a car that responds by restricting power, which can be genuinely awkward on faster roads where you may not be able to accelerate as you expect. If the car has gone into a reduced-power mode, plan the journeys around that rather than discovering it during an overtake.

Will it fail an MOT?

Beyond the lamp, which is a major defect and enough on its own, this code has a specific complication at test time. The petrol emissions procedure requires the engine to be held at fast idle, typically between 2,500 and 3,000 rpm, and the diesel smoke test requires it to be revved to its governed maximum several times. A car that has dropped into a reduced-power mode because of this fault may simply not do either, and an emissions test that cannot be carried out is recorded as a major failure in the same way as a test that is carried out and failed. That is worth knowing because it changes the advice: with most codes you can weigh up whether to fix it before the test, but with this one there may be nothing to weigh up.

The light this turns on

Engine management lightThe engine management system has found a fault it cannot correct and has stored a code for it. The car will usually still drive, but an illuminated engine light is an automatic MOT failure on petrol cars first used from 1 July 2003 and diesels from 1 July 2008.

Common questions

What is the difference between P0404 and P0401?: P0401 is about gas: the control unit cannot detect enough exhaust being recirculated. P0404 is about the valve's own movement: the control unit commanded a position, the valve's internal position sensor reported a different one, and nothing is said about whether gas actually flowed. They share causes, chiefly carbon build-up, but the diagnosis differs. P0401 is confirmed by looking at flow effects, P0404 by comparing commanded against actual valve position on a diagnostic tool.
Will P0404 fail an MOT?: Yes. The engine management lamp it illuminates is a major defect on petrol cars first used on or after 1 July 2003 and diesels from 1 July 2008. There is an additional risk with this particular code: some cars respond to it by entering a reduced-power mode, and if the engine cannot be held at fast idle for the petrol gas test or revved out for the diesel smoke test, the emissions test cannot be completed, which is recorded as a major failure in its own right.
Can a P0404 code cause limp mode?: Yes, on many vehicles. If the control unit cannot verify where the recirculation valve is sitting, it cannot predict how much exhaust gas is entering the engine, and the safe response on a turbocharged engine in particular is to restrict boost and power until the fault is resolved. Not every car does this — plenty run entirely normally with the code stored — but where it happens the loss of performance is deliberate rather than a symptom of damage.
Do I need to replace the EGR valve for P0404?: Not always, but more often than for the flow codes. If carbon is binding the pintle, a thorough clean can restore normal movement. If the position sensor inside the valve has worn, cleaning achieves nothing because the mechanism was moving correctly all along — and on high-mileage engines both problems tend to arrive together. Have the commanded and actual positions compared first, and check the connector and wiring, since heat damage there mimics a failed valve exactly.

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