P0406: EGR Sensor A Circuit High

What does P0406 actually mean?

This code sets when the EGR position sensor's signal voltage reads above the maximum the computer expects, typically from an open circuit or a short to a reference voltage rather than a real position reading. The computer treats this the same as a missing sensor.

The code sits in the eGR position sensing area and is reported by the eGR system monitor. It names the circuit or the condition where the computer saw a reading outside its expected range. It does not name a failed part, and several genuinely different faults set this same code, which is the whole reason the causes below are ordered rather than listed.

What people usually notice

• Check engine light
• Failed emissions test
• Occasional rough idle

Why does P0406 set?

The causes below are ordered by how often they turn out to be the answer, cheapest and most common first. That order matters more than the list itself: the expensive component named in the code text is very rarely the first thing to replace, and working down from the top is what stops a eGR position sensing fault becoming an expensive guess.

Open or broken wire in the sensor circuit is the one to rule out before anything else. A broken wire is the most direct cause of a signal stuck high.

The order worth checking on P0406

Start with freeze frame. Engine load and RPM at the fault show under what condition the high reading occurred. That snapshot is taken at the instant the code set, and it is the single most useful piece of evidence you have, which is also why clearing the code before reading it is such a costly habit.

This code is diagnosed by watching values change rather than by reading the number again, so the tool has to stream live data and hold freeze frame. Live EGR position sensor voltage at rest and through a commanded sweep confirms whether the signal is pinned high or missing entirely.

The order worth working in

• 1. Open or broken wire in the sensor circuit: A broken wire is the most direct cause of a signal stuck high.
• 2. Wire shorted to a reference voltage source: A chafed wire touching a reference wire pushes the signal high.
• 3. Corroded connector creating high resistance: Corrosion can raise resistance enough to look like an open circuit.
• 4. Failed EGR position sensor: The sensor element itself can open internally with age.
• 5. Failed computer input circuit: Less common, but a damaged input stage inside the computer produces the same symptom.

Is it safe to drive with P0406?

Safe to keep driving, since this reflects a sensor circuit fault rather than a mechanical or overheating risk.

This is a severity 2 of 4 code. It will not usually strand you, but it will fail an emissions test while it is present, and the underlying fault rarely gets cheaper by being left alone.

What does it cost to ignore P0406?

Without a trustworthy position signal the computer cannot confirm EGR flow, which typically leads to a failed emissions test until the circuit is repaired.

There is also the diagnostic cost of waiting. Freeze frame data, pending code history and the readiness monitor state all describe the fault as it was when it set, and all of it is lost the moment somebody clears the code to make the lamp go out.