What P0406 means
P means powertrain (engine and transmission). The 0 marks a generic code defined by SAE, so the same number means the same thing across makes. The third character, 4, places it in the auxiliary emission controls subsystem; the last two digits identify the specific fault.
Many EGR valves include a position sensor so the engine computer knows how far the valve actually opened. "A" and "B" identify which sensor, as defined by the manufacturer.
The letter "A" distinguishes between two or more of the same component on one vehicle. Which physical part is "A" is set by the manufacturer, so confirm it in the service information before replacing anything.
What “Circuit High” means here
"High" means the signal or control voltage rose above the maximum the module expects. On a three-wire sensor fed by a 5 V reference, typical causes are a signal wire shorted to the reference or to battery voltage, an open sensor ground, or a failed sensor.
What you may notice
Symptoms depend on the vehicle and on how the module responds to the fault. Any of these can accompany P0406:
- Often none
- EGR flow codes stored alongside
- Rough idle if the valve is not where the module thinks it is
Where to start diagnosing P0406
- Check the 5 V reference, ground and signal at the valve connector.
- Watch EGR position while commanding the valve open and closed, where the scan tool allows.
- Inspect for carbon preventing full valve travel.
Clear the code only after recording freeze frame data: it shows the engine speed, load and temperature at the moment the fault set, which is often the best clue to reproducing it.
Live data to watch
These standard OBD2 values relate to P0406. Support varies by vehicle; a value your car does not report will simply be missing.
| Value | What to look for |
|---|---|
| Commanded EGR (PID 0x2C) | Compare with reported position. |
| EGR error (PID 0x2D) | Should stay small. |
Related codes
Codes from the same EGR position / feedback sensor family. If several are stored together, the shared part — wiring, a common reference voltage, or the component itself — is the place to start.
Further reading
Frequently Asked Questions
What does P0406 mean?
P0406 is the generic SAE J2012 code for "Exhaust Gas Recirculation Sensor "A" Circuit High". In plain terms, it means the engine computer has detected a "circuit high" fault on the EGR position / feedback sensor.
Can I keep driving with P0406?
Usually drivable; have it repaired to keep the EGR system working.
What should I check first for P0406?
Check the 5 V reference, ground and signal at the valve connector.
Definition source: SAE J2012 / ISO 15031-6 generic DTC definitions. This is the generic meaning that applies across makes; a manufacturer can define a code more narrowly or add its own sub-codes, so confirm against the service information for your vehicle before replacing parts. Written by the OBDAssistant engineering team — see our editorial policy.