What does P0402 mean?

Too much exhaust gas is recirculating into the intake.

Why does a stuck-open EGR valve make the engine idle badly?

It lets exhaust gas dilute the mixture at idle, where the engine needs almost none.

Emission control

P0402 — Exhaust Gas Recirculation Flow Excessive Detected

More exhaust gas is flowing back into the intake than the ECU expects, usually because the EGR valve is stuck open or leaking. The engine can idle badly or stall.

Severity: Medium

Common causes

EGR valve stuck open or not seating
Faulty EGR position sensor
Vacuum control problem
Carbon holding the valve open
Faulty EGR solenoid

Symptoms

Rough idle or stalling at idle
Hesitation at low speed
Surging
Check engine light

How to check

Check whether the EGR valve closes fully at idle
Inspect the vacuum line and solenoid
Watch the EGR position feedback on a scan tool
Clean or replace the valve

Can you keep driving?

Drivable, but stalling at idle is possible. Repair it soon.

Frequently asked questions

What does P0402 mean?: Too much exhaust gas is recirculating into the intake.
Why does a stuck-open EGR valve make the engine idle badly?: It lets exhaust gas dilute the mixture at idle, where the engine needs almost none.

General reference information. It does not replace a diagnosis on your own vehicle. The same code can have different causes depending on make, model year and engine; check the manufacturer's documentation or ask a technician with a scan tool.