P0402: OBD-II code P0402 flags excessive EGR flow — the EGR valve is admitting more exhaust than the PCM commanded, causing rough running and driveability issues.

Symptoms

• Check engine light is on; rough idle, stumble, or stalling because too much exhaust is diluting the intake charge.
• Hesitation or loss of power under light throttle where EGR is normally most active.
• The vehicle may fail an OBD-II emissions test until the EGR monitor completes clean.

Common causes

• A stuck-open EGR valve, often due to carbon buildup preventing the valve from seating fully.
• A ruptured EGR valve diaphragm or damaged pintle on vacuum-controlled designs.
• A leaking EGR gasket, cracked EGR tube, or a stuck-open EGR solenoid on electronic systems.

What to do?

Command the EGR closed with a scan tool at idle — if the idle doesn't smooth out, the valve is stuck open. Remove the EGR valve and inspect for carbon; clean or replace it. Check the EGR gasket and tube for leaks. Replace the solenoid if the valve is electronically controlled and stuck.