The P0402 code means the computer detected exhaust gas recirculation flow excessive — more exhaust re-entering the engine than commanded. Either the EGR valve is genuinely stuck open, usually by carbon buildup, or a flow sensor is exaggerating. At idle, that extra inert gas makes the engine stumble and stall.

P0402 — “Exhaust Gas Recirculation Flow Excessive Detected” — is the code for a well-intentioned valve that forgot how to stop. Exhaust gas recirculation exists because combustion runs hot enough to cook nitrogen into NOx pollution; feeding a measured trickle of inert exhaust gases back into the intake at cruise cools the burn and cuts NOx — emission control that costs nothing in engine performance when it behaves. The system’s whole discipline is measured and timed — some flow at cruise, zero at idle. P0402 means the computer caught flow running past the menu: exhaust recirculating when the engine needs clean air most, which is exactly when you feel it.

P0402 exhaust gas recirculation flow trouble is an idle story — that’s why the symptom list reads like an idle complaint rather than a highway one. At speed the extra flow disappears into a big airflow number; at a stoplight it’s a meaningful slice of what the cylinders inhale, and the engine answers with rough idling, hesitation just off idle, and stalling as the revs fall. A mechanic hears “runs fine on the road, dies at stops” and this code family comes to mind before the scanner is even out of the drawer.

Two suspects, one honest question

Every P0402 diagnosis is the same question: is the flow real, or is the witness lying? Real excessive EGR flow is carbon’s work — deposits wedge the EGR valve’s pintle open a crack, or clog the EGR passages until the pressures mislead the system. The lying witness is a flow sensor, and here Ford wrote the textbook: the DPFE sensor that generations of Taurus and F150 engines used to measure EGR drifts with age until it can indicate a flood through a closed valve. To diagnose it, the scanner splits the two in minutes — commanded versus actual readings while the engine is running, then a look at the sensor’s voltage against reality. Then the garage work is ordinary and satisfying: inspect the EGR valve, clean or replace it, clear the carbon from the passages, or fit the new sensor — and prevent the rerun by fixing whatever let the carbon build, because a short-trip engine that never gets hot soots everything faster. Our emissions side and engine diagnostics share this code’s casework for exactly that reason.

The family resemblances are worth knowing: unresolved EGR trouble raises combustion temperatures and increased emissions that age the catalytic converter — P0420’s territory — and an EGR leak past the seat at idle can stumble cylinders into the misfire counters of P0300. Codes rarely travel alone; the order you fix them in is most of the craft.

Measured first, repaired once

Code P0402 rewards the shop that measures and embarrasses the one that guesses — the same error codes clear for a week after any parts swap, then return on the next drive cycle to resolve nothing. Here the diagnosis is ordinary automotive discipline — shown on the scan data, the price is agreed before work begins — in writing when the job is major, and the fix matches the actual failing part: sensor, valve, passages, or the cleaning that makes new parts unnecessary. Book the diagnosis — an idle you can trust at every stoplight is a modest repair away, and your engine’s warranty on our work comes in writing.