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.
Frequently asked questions
What is the most common cause of P0406?
The most likely cause is: Open signal or ground wire. Several other faults can set P0406 as well, so confirm the real cause with live data and the checks above before replacing any parts.
Can I just clear P0406 and keep driving?
Clearing P0406 with an OBD2 scanner turns the warning light off, but if the underlying fault has not been repaired the code will usually come back within a few drive cycles. Fix the cause first, then clear the code to confirm the repair worked.
How serious is P0406?
P0406 is a moderate-severity code. The car is often still drivable, but have it diagnosed soon — the underlying problem can get worse and it may fail an emissions test.
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