P0406 — Exhaust Gas Recirculation Sensor A Circuit High
Moderate severityWhat it means
P0406 is stored when the engine control unit sees a fault involving the exhaust gas recirculation (EGR) system. The EGR system routes a little exhaust gas back into the intake to lower combustion temperature and NOx emissions. A “high” reading means the signal is pinned near the top of its range — often an open circuit, a lost ground, or a short to voltage.
Common symptoms
- The live reading sits abnormally high or maxed out
- Pinging or knock under load
- Rough idle or stalling
- Failed emissions test
Likely causes
- Open signal or ground wire
- Signal wire shorted to a voltage supply
- Carbon-clogged EGR valve or passages
- Faulty EGR valve, solenoid or position sensor
What to check first
- Check for an open circuit or missing ground at the sensor connector
- Remove and inspect the EGR valve and passages for carbon
- Command the valve with a scan tool and watch the engine response
Can you keep driving?
Often drivable, but pinging under load can damage the engine over time — avoid heavy loads until repaired.
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