⚠ P0402 MEDIUM SEVERITY

P0402: what it means

Officially? A wall of jargon. To most people? Absolutely nothing. That's exactly why we built the app.

The official definition vs. an actual explanation

Plug into any code reader and you'll get the same line every other app shows you. Technically correct. Practically useless.

What other apps tell you

"P0402: Too much exhaust gas is recirculating through the EGR valve."

What OBD Mechanic tells you

"The EGR system is letting more exhaust gas back into the intake than the ECU asked for. Usually the EGR valve isn't closing properly, most often because carbon has built up and it's stuck partly open. Too much exhaust in the intake makes the engine run rough, especially at idle. It can hesitate, stall when you come to a stop, and on diesels blow black smoke. Start with a clean of the valve. On older vacuum-operated systems, check the vacuum hose and control valve too. On many diesels the ECU judges EGR flow from the airflow sensor, so a MAF reading low can set this code even with a good valve."

What actually causes it, cheap to dear

Worth checking in roughly this order. Don't jump straight to the expensive fix:

EGR valve stuck partly open with carbon, the most common cause
Faulty EGR valve position sensor or motor, so the valve doesn't do what it's told
Vacuum control valve or hose fault on older vacuum-operated systems
Airflow (MAF) sensor reading low, on cars that use it to work out EGR flow
Faulty sensor the car uses to measure EGR flow (MAP or a pressure sensor, depending on the car)

usually cheap depends pricey

Suggested checks

With the engine cold, remove and inspect the EGR valve. If it's caked in carbon and won't close fully, clean it with carb or EGR cleaner
Check the valve moves freely and closes fully
On a vacuum-operated valve, check the small vacuum hoses and control valve
On a diesel, clean the MAF with proper MAF cleaner too. It's cheap and can be part of the picture
If cleaning doesn't hold, the valve itself is the likely buy

Which dials to watch for P0402

The code itself is a snapshot. The fault sets after the car's watched the numbers over a whole drive cycle. Here's what's worth keeping an eye on:

Cmd EGR

Shows what the ECU is asking for. If it's near zero but the engine still runs rough at idle, the valve isn't closing like it's being told.

Not simulated live Our free demo drive doesn't model this reading yet. Watch it live on your own car in the app.

MAF (live graph)

On many diesels, airflow drops when EGR opens because exhaust takes the place of fresh air. If MAF stays low even when Cmd EGR falls to nothing, exhaust is still getting through, or the MAF itself is reading low.

Live simulation on the readings our free demo drive supports. This is how the app’s dials move.

It spots things you'd never see in the numbers

Codes rarely tell the whole story. The clever part is reading them together, and against your exact car. Here's the kind of thing it catches:

AI Mechanic

What our mechanic says

P0402

The EGR system is letting more exhaust gas back into the intake than the ECU asked for. Usually the EGR valve isn't closing properly, most often because carbon has built up and it's stuck partly open. Too much exhaust in the intake makes the engine run rough, especially at idle. It can hesitate, stall when you come to a stop, and on diesels blow black smoke. Start with a clean of the valve. On older vacuum-operated systems, check the vacuum hose and control valve too. On many diesels the ECU judges EGR flow from the airflow sensor, so a MAF reading low can set this code even with a good valve.

P0402: quick questions

Can I still drive with P0402?

The engine may run rough, lose power or stall at idle. Get it checked before you drive far, and take care at junctions and roundabouts where a stall would leave you stuck.

Is P0402 the opposite of P0401?

Yes. P0401 means not enough EGR flow, usually blocked passages. P0402 means too much, usually a valve stuck open. Carbon is often behind both, so a clean is a sensible first step either way.

How much will it cost to fix?

A clean costs a can of cleaner and some time, or modest labour at a garage. A new EGR valve is often somewhere around £80 to £250 for a branded part depending on the car, plus fitting. Some diesels bury the valve, which adds labour.

Can I just block off the EGR?

No. Removing or blanking the EGR is illegal for road use in the UK. It also tends to bring up other fault codes. Clean or replace it properly.