Why does my car stall with a P0402 code?

Because exhaust gas is entering the inlet at idle, when it should not be. Recirculated exhaust contains almost no oxygen, so it displaces the fresh air the engine needs at the point when it is drawing the least air anyway. The mixture becomes too diluted to burn cleanly, the idle drops and the engine cuts out. The same fault is barely noticeable at higher revs because the proportion of exhaust gas to fresh air is far smaller.

Will P0402 fail an MOT?

Yes, and potentially twice over. The illuminated engine management lamp is a major defect for petrol cars first used on or after 1 July 2003 and diesels from 1 July 2008. Beyond that, the emissions test needs the engine to hold a stable idle for the readings to be valid, and a car that hunts or stalls may prevent the tester completing the test at all, which is itself a major failure. An unstable idle also raises hydrocarbon readings.

Is P0402 the same as P0401?

They are opposites within the same system. P0401 means the control unit cannot detect enough exhaust gas being recirculated, usually because carbon has blocked the valve or its passages shut. P0402 means it detects more than it commanded, usually because carbon is holding the valve open so it cannot seal. The parts involved overlap heavily, but the symptoms differ sharply: insufficient flow is often unnoticeable, while excessive flow makes the idle lumpy and can stall the engine.

Can I drive with an EGR valve stuck open?

Short journeys are acceptable, but do not treat it as something to live with. Nothing is being damaged rapidly, so the risk is a practical one: a car that cuts out as you pull away from a junction is unsafe in traffic, and you lose power steering assistance the instant the engine stops. If it is stalling regularly, get it dealt with rather than working around it.

Powertrain code · standard

P0402Too much exhaust gas being recirculated

Standard definition: Exhaust Gas Recirculation Flow Excessive Detected

Last updated 27 July 2026

Quick answer

The opposite of P0401: exhaust gas is getting back into the engine when it should not be, usually because carbon is stopping the valve seating properly. Unlike most emissions codes, this is one you can feel — it makes the idle lumpy and can stall the engine at junctions.

How the car works this out

The control unit uses the same checks as for insufficient flow but reads them the other way round. It sees a drop in fresh airflow, a rise in intake temperature or a pressure difference across the recirculation path at moments when the valve should be closed, or a larger change than commanded when it is open. Since the valve should sit fully shut at idle and under hard acceleration, gas found flowing at those points is unambiguous.

What you might notice

A lumpy, uneven or hunting idle that improves as soon as the engine is revved
Stalling at junctions, when coming to a stop or when the air conditioning cuts in
Hesitation and a flat spot pulling away from rest
Poor running when cold, clearing up as the engine warms
On a diesel, a puff of smoke and a slightly acrid smell at idle

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 holding the valve off its seat so it never fully closesCommon
How to check: The idle is the tell. Disconnect the valve's electrical or vacuum control so it defaults closed; if the idle cleans up immediately, the valve is passing gas it should not.
2A failed or worn valve that no longer seals, including a split vacuum diaphragmCommon
3A vacuum control solenoid stuck applying vacuum, so the valve is held open continuouslyPossible
How to check: Unplug the solenoid. If the valve closes and the idle recovers, the solenoid rather than the valve is at fault.
4A cracked or leaking recirculation cooler or its gasket, letting exhaust gas past with the valve shutPossible
5A differential pressure sensor over-reading, or its two sensing hoses fitted the wrong way round after earlier workPossible
How to check: With the engine off the sensor should report essentially no pressure difference. A reading well away from zero at rest, or a negative one, points at the sensor or its plumbing rather than the valve.
6The control unit commanding excessive flow because an airflow or manifold pressure signal is wrongLess common

What to do about it

1If the car is stalling at junctions, treat it as a priority rather than a nuisance. An engine that dies as you pull out of a side road is a genuine hazard, and you also lose power steering assistance the moment it stops.
2Remove and inspect the valve. A seat carboned enough to hold the valve open is visible immediately, and cleaning it is often the whole repair.
3Check the sensing hoses if the system uses a differential pressure sensor. Hoses reversed during earlier work is a surprisingly common cause and costs nothing to correct.
4Look at the cooler and its gasket on diesels. A cracked cooler passes gas with the valve perfectly shut, so a cleaned valve will not fix it.
5Have the idle checked after the repair. The relearn on some cars needs a settling period before the idle returns to normal.

Can you keep driving?

Briefly, gently

Mechanically the fault is not damaging anything quickly, but the driving consequences are the point. A car that stalls when you come to a stop or pull away is unpredictable in traffic, and restarting at a busy junction is exactly where you do not want to be. If the idle is merely lumpy and it never actually cuts out, ordinary driving is fine until it can be looked at.

Will it fail an MOT?

The lamp fails the car as a major defect, but this code has a second and less obvious route to a failure certificate. The petrol emissions test includes a measurement taken at natural idle, and the diesel smoke test requires the engine to be revved from a stable idle several times. A car whose idle is lumpy, hunting or dropping to a stall cannot always hold steady long enough for the analyser to take a valid reading, and an emissions test that cannot be completed is a major failure in its own right. A misfiring idle also throws hydrocarbons, so where the reading does come through it may fail on the numbers. This is one of the few emissions codes where the tester's difficulty, rather than the exhaust chemistry, is what ends the test.

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

Why does my car stall with a P0402 code?: Because exhaust gas is entering the inlet at idle, when it should not be. Recirculated exhaust contains almost no oxygen, so it displaces the fresh air the engine needs at the point when it is drawing the least air anyway. The mixture becomes too diluted to burn cleanly, the idle drops and the engine cuts out. The same fault is barely noticeable at higher revs because the proportion of exhaust gas to fresh air is far smaller.
Will P0402 fail an MOT?: Yes, and potentially twice over. The illuminated engine management lamp is a major defect for petrol cars first used on or after 1 July 2003 and diesels from 1 July 2008. Beyond that, the emissions test needs the engine to hold a stable idle for the readings to be valid, and a car that hunts or stalls may prevent the tester completing the test at all, which is itself a major failure. An unstable idle also raises hydrocarbon readings.
Is P0402 the same as P0401?: They are opposites within the same system. P0401 means the control unit cannot detect enough exhaust gas being recirculated, usually because carbon has blocked the valve or its passages shut. P0402 means it detects more than it commanded, usually because carbon is holding the valve open so it cannot seal. The parts involved overlap heavily, but the symptoms differ sharply: insufficient flow is often unnoticeable, while excessive flow makes the idle lumpy and can stall the engine.
Can I drive with an EGR valve stuck open?: Short journeys are acceptable, but do not treat it as something to live with. Nothing is being damaged rapidly, so the risk is a practical one: a car that cuts out as you pull away from a junction is unsafe in traffic, and you lose power steering assistance the instant the engine stops. If it is stalling regularly, get it dealt with rather than working around it.

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