Category: Powertrain → EGR System
What it means (plain English)
The ECM/PCM sees a voltage signal higher than expected from the EGR position sensor (often >4.5V). It thinks the EGR valve is wide open, or the circuit is shorted to voltage. (obd-codes.com)
Symptoms you’ll notice
- Check Engine Light (repairpal.com)
- Rough idle, poor acceleration, or stalling
- Rich running (too much EGR assumed)
- Black smoke on diesels (excess exhaust dilution)
- Reduced power or limp mode
Priority level
Medium–High. Incorrect sensor feedback disrupts EGR control and can create drivability/emissions problems.
Common causes
- Faulty EGR valve/position sensor (sensor shorted high)
- Short to power on the signal wire
- Broken ground reference
- Connector corrosion or bent pins
- ECM fault (rare) (yourmechanic.com)
How pros diagnose it
- Scan PID: observe EGR position voltage (shouldn’t be pegged >4.5V).
- Back-probe: verify 5V ref, ground, and signal return.
- Inspect harness/connector for shorts to power.
- Manually move EGR valve (if possible) and watch voltage sweep.
- If signal remains high regardless of valve movement, sensor is failed.
Likely fixes
- Replace EGR valve/position sensor assembly
- Repair/replace wiring harness (remove short-to-power condition)
- Clean connector pins or re-pin as needed
- ECM replacement only if verified (rare)
Related / companion codes
- P0404 (Range/Performance)
- P0405 (Circuit Low)
- P0401/P0402 (Flow faults)
Tech notes
High-mileage diesels often carbon up the EGR valve, which can confuse the position sensor. Cleaning sometimes resolves P0406, but sensor/valve replacement is the more reliable long-term fix.