OK short-term; fix soon to stop stalling, pass emissions. P0402 means the powertrain control module has detected more exhaust gas recirculation (EGR) flow than it commanded — typically an EGR valve stuck or leaking open, or a pressure-feedback sensor reporting excessive flow at idle.
What P0402 means
The EGR system routes a metered amount of inert exhaust gas back into the intake to lower peak combustion temperatures and cut NOx emissions. The control module opens the EGR valve only under specific load conditions and expects it to seal completely at idle, when no recirculation is wanted. To verify this, the EGR monitor watches a flow-feedback signal — on many Ford applications a differential pressure feedback EGR (DPFE) sensor, on other designs a valve-position sensor or a manifold-pressure (MAP) delta. The module stores a baseline reading with the engine off and the valve closed, then compares it to the reading at idle. If the idle signal exceeds the stored key-on-engine-off value by a calibrated amount (for example a DPFE PID roughly 0.5 volt higher than baseline), the module concludes the valve is passing exhaust when it should be shut and sets P0402. Because unwanted EGR at idle dilutes the intake charge, the fault often shows up first as a rough, unstable idle before the check-engine light appears.
Symptoms
- Illuminated check-engine / malfunction indicator lamp
- Rough, unstable, or lopey idle as exhaust dilutes the intake charge
- Hesitation, stumble, or stalling at idle and low speeds
- Reduced power and occasional light knock or pinging under load
- Failed emissions test due to the active EGR monitor fault
Common causes
- EGR valve stuck or leaking open, or a valve pintle not seating fully
- Carbon buildup holding the valve off its seat or restricting the EGR passages
- Faulty, slow-responding, or contaminated EGR flow sensor (DPFE or valve-position sensor) reporting excessive flow
- EGR vacuum control error — plugged vacuum regulator solenoid vent, damaged EVR solenoid, or a short-to-ground in its control circuit
- Vacuum leaks, cracked or misrouted vacuum hoses, or blocked/leaking EGR feedback pressure lines
Severity & driving advice
Severity: Moderate — Drivable, but unwanted EGR at idle causes rough running and stalling, and the vehicle will fail an emissions test until repaired.
Can I drive? OK short-term; fix soon to stop stalling, pass emissions.
Diagnostic approach
- Scan and record freeze-frame data — Retrieve P0402 along with any companion EGR or emissions codes and note the conditions when it set. Confirm whether the fault appears at idle, since P0402 is usually flagged during the idle undesired-flow test.
- Compare the flow-feedback signal at KOEO versus idle — On DPFE-equipped systems, view the sensor PID with the key on and engine off, then at warm idle. A reading roughly 0.5 volt higher at idle than the engine-off baseline, or a persistently rough idle, points to a genuine stuck-open or leaking valve rather than a false alarm.
- Inspect and manually test the EGR valve — Remove the valve and check the pintle and seat for carbon deposits that prevent full closure. On vacuum-actuated designs verify the diaphragm holds vacuum and closes fully; on electronic valves confirm it returns to the commanded closed position and check the passages and EGR tube for restriction or leaks.
- Check the vacuum control and feedback plumbing — Inspect the EGR vacuum regulator (EVR) solenoid, its vent, and all hoses for blockage, cracks, or incorrect routing. Verify the feedback pressure lines to the DPFE sensor are clear and leak-free, since a plugged or leaking hose skews the flow calculation.
- Verify the sensor and its wiring, then confirm the repair — Back-probe the EGR flow sensor for correct reference voltage, ground, and signal, and check the EVR control circuit for a short to ground. After the suspect part is replaced or cleaned, clear the code and complete a drive cycle to confirm P0402 does not return.
Make & model notes
Ford: Many Ford applications (F-Series, Crown Victoria, E-Series, Mustang, Grand Marquis, Town Car) use a differential pressure feedback EGR (DPFE) sensor with a vacuum-operated valve and an EGR vacuum regulator (EVR) solenoid. P0402 typically sets when the DPFE reading at idle exceeds the key-on-engine-off baseline by a calibrated amount — commonly a stuck-open valve, a plugged EVR vent, a restricted EGR tube, or a slow/damaged DPFE sensor. A DPFE PID about 0.5 V higher at idle than at KOEO, or a rough idle, indicates a hard fault.
Chrysler: On Chrysler/Stellantis vehicles that still use EGR, the system is electronically controlled and flow is inferred from manifold pressure and valve position rather than a separate DPFE sensor. Suspect a carbon-fouled valve that won't seat, a sticking pintle, or a position-sensor fault. Note that many later Chrysler gasoline engines omit EGR entirely, so confirm the engine is actually EGR-equipped before chasing this code.
FAQ
What is the difference between P0402 and P0401?
They are opposites. P0401 means the module detected too little EGR flow (valve stuck closed or passages clogged), while P0402 means it detected too much flow — usually a valve stuck or leaking open, or a feedback sensor reporting excess flow when the valve should be shut.
Can I drive with a P0402 code?
Short trips are usually possible, but excess EGR at idle can cause rough running, hesitation, and stalling, which is inconvenient and can be unsafe in traffic. It will also fail an emissions inspection, so it should be repaired promptly.
Will cleaning the EGR valve fix P0402?
Often yes. Carbon buildup that keeps the valve pintle off its seat is a common cause of excessive flow. Removing the valve and cleaning the pintle, seat, and passages frequently restores a proper seal — but if the valve or its sensor is worn or damaged, replacement is needed.
My car doesn't seem to have an EGR valve — why the P0402?
Many modern gasoline engines use internal or cooled EGR strategies, and some omit EGR altogether. If yours lacks a conventional valve, the code most likely reflects a faulty flow or pressure feedback sensor, a wiring fault, or a software/monitor issue rather than a stuck valve. Confirm the exact EGR design for your engine before replacing parts.