Category: Powertrain → EGR System
What it means (plain English)
The ECM/PCM commanded the EGR system, but detected too much exhaust gas flow entering the intake. In other words, the EGR valve is allowing more exhaust into the engine than it should, which can cause stalling, rough running, and increased emissions.
(obd-codes.com)
Symptoms you’ll notice
- Check Engine Light on (repairpal.com)
- Rough idle or engine stalling at stoplights
- Hard starting, especially when warm
- Hesitation or stumble during acceleration
- Black smoke on diesels (rich condition)
- Failed emissions test due to high HC/CO levels (yourmechanic.com)
Priority level
Medium–High. While not as dangerous as turbo overboost, excessive EGR flow can cause serious drivability problems and raise emissions.
Common causes
- EGR valve stuck open (carbon build-up or failed solenoid/actuator) (obd-codes.com)
- Faulty EGR control solenoid (vacuum type systems)
- Faulty DPFE sensor misreporting flow (Ford-specific issue)
- EGR position sensor / wiring issues (on electronic valves)
- EGR passages restricted in a way that confuses sensors
- ECM/PCM software calibration issue (rare)
How pros diagnose it (step-by-step)
- Scan tool test — look at EGR commanded vs actual position or flow.
- Idle behavior test — if the EGR valve is stuck open, the engine will run rough or stall at idle.
- Vacuum test (if applicable) — check that the valve diaphragm holds vacuum and isn’t stuck open.
- Bench inspection — remove the EGR valve and inspect for carbon preventing closure.
- Sensor checks — test DPFE/EGR position sensor voltage sweep and compare to spec.
- Wiring check — inspect connectors and harness near hot exhaust paths for corrosion, shorting, or melting.
- ECM command test — command the valve closed with a scan tool and observe if idle smooths out.
Likely fixes
- Clean or replace EGR valve
- Replace DPFE sensor (common Ford fix)
- Replace EGR solenoid or actuator
- Repair wiring/connectors for sensor or valve
- Reflash/update ECM software if an OEM bulletin applies
Related / companion codes
- P0401 — EGR Flow Insufficient
- P0404 — EGR Range/Performance
- P0405/P0406 — EGR Sensor Circuit Low/High
- Misfire codes (P0300+) if stalling occurs
Tech notes (quick hits)
- On Ford and Chrysler products, DPFE sensor failure is a leading cause of false P0402 readings.
- On diesels, EGR valves often stick open from soot build-up, leading to too much recirculation and poor drivability.
- A quick test is to temporarily block off the EGR passage (for diagnosis only) to see if idle smooths out — confirming excess EGR flow.