P0406 — Exhaust Gas Recirculation Sensor A Circuit High

OBD2 code P0406 flags a problem with the exhaust gas recirculation (EGR) system (exhaust gas recirculation sensor a circuit high). The car is usually drivable while you plan the repair. Common culprits: ECM control circuit fault, faulty EGR solenoid or wiring, failed EGR valve or position sensor.

powertrain (engine/transmission)

Is it safe to drive?

Diagnose promptly P0406 is generally not an immediate stop-driving condition, but an incorrect high signal can affect EGR control, emissions, idle quality, and fuel economy. Continue cautiously if the engine runs normally, and avoid further driving if it stalls, runs severely rough, overheats, or produces a strong exhaust odor.

Typical symptoms

The check-engine light illuminates and P0406 appears in stored or pending trouble codes
The engine may run slightly rough or hesitate during light acceleration
Fuel economy may decrease because EGR operation is inaccurate or disabled
Some vehicles show a mild loss of power or intermittent idle instability
P0406 may be the only noticeable symptom when the engine otherwise runs normally

Common causes

A failed EGR sensor A with an internally high or out-of-range signal output
A shorted EGR sensor A signal wire to the 5-volt reference or another power circuit
Corrosion, water intrusion, or damaged terminals in the EGR sensor A connector
A high-resistance ground or damaged wiring in the EGR sensor A circuit
A rare fault in the control module's EGR sensor reference or input circuit
On vehicles using differential-pressure EGR sensing, an exhaust leak can produce a misleading high reading

How to diagnose with an OBD scanner

Confirm P0406 with an OBD2 scan Connect an OBD2 scanner or ELM327 adapter with a scan tool such as AIScanAuto. Confirm P0406, review freeze-frame data, and, if supported, compare the live EGR Sensor A reading with the commanded EGR position. Save the data before disconnecting the battery or clearing codes.
Inspect the EGR sensor A and connector Locate EGR sensor A and examine its connector and wiring near hot or moving components. Look for water, green corrosion, bent terminals, pin pushback, chafing, or a connector that is not fully latched.
Test reference, ground, and signal circuits Using the vehicle wiring diagram and a high-impedance meter, back-probe the sensor's 5-volt reference, ground, and signal pins. Confirm that the reference and ground meet manufacturer specifications and that the signal is not above the calibrated range. Perform a wiggle test while watching the scan data.
Isolate the sensor from the harness With the ignition appropriately enabled and without shorting any pins, disconnect EGR sensor A and observe whether the abnormal high signal disappears. A normal disconnected signal points toward the sensor; a persistently high signal points to a harness short or control-module circuit fault.
Verify EGR flow after the electrical checks If the sensor circuit passes, inspect the EGR valve or passages for carbon restriction and perform the manufacturer-specific EGR flow test. For a differential-pressure sensor, also inspect nearby exhaust joints for leaks that could distort the reading.
Repair and verification Repair the identified wiring or connector, or replace EGR sensor A with the correct part, then clear P0406 and complete a drive cycle. Recheck for stored, pending, and permanent codes. If P0406 returns with correct reference voltage and wiring, perform control-module diagnosis.

Approximate repair cost

A typical independent-shop range for P0406 diagnosis and repair is about $150-$700, depending on whether the fault is the sensor, connector, wiring, EGR valve, or an uncommon control-module circuit. Figures are estimates and vary by region.

Common questions

What does P0406 mean?

P0406 means the control module sees the EGR Sensor A signal as higher than the calibrated allowable voltage for that vehicle.

Can I keep driving with P0406?

Usually, yes for a short distance if the engine runs smoothly, because P0406 is not normally an immediate stop condition; prompt diagnosis is still advisable.

Does P0406 always mean the EGR sensor is bad?

No. P0406 can also result from a shorted signal wire, a corroded connector, a poor ground, an exhaust leak, or a rare control-module fault.

Will clearing P0406 fix the problem?

No. Clearing the code only resets its monitoring status; the high EGR Sensor A signal must be diagnosed, repaired, and verified.

The EGR system routes a measured portion of exhaust gas back into the intake to lower combustion temperatures and cut NOx emissions. The PCM drives an EGR valve that opens and closes according to engine temperature, load, and RPM. A position sensor integrated with or mounted on that valve continuously reports the valve's actual opening to the computer.

When that sensor's voltage signal climbs well above its normal operating range—often hovering near battery voltage—the PCM recognizes the reading as invalid. After verifying the fault across several consecutive drive cycles, the computer stores P0406 and turns on the Check Engine light. The freeze frame captured alongside the code shows engine RPM, load, temperature, and speed at the moment of detection, which pinpoints the driving conditions most likely to trigger the problem.

"Circuit high" points to one of two root causes: either the signal wire has shorted to a power source, forcing voltage upward, or the ground return has opened, allowing the signal to float without restraint. In either scenario the PCM can no longer trust the position feedback, so EGR operation becomes unreliable or shuts down entirely.

Common Symptoms of P0406

Check Engine light remains illuminated
Rough idle, occasional stalling, or unstable engine operation
Pinging or knocking during acceleration or under load
Hesitation and flat spots when throttle is applied
Noticeable drop in engine power
Elevated NOx emissions and failed emissions tests
On certain models, reduced power mode or limp-home activation

Common Causes of P0406

Signal wire shorted to battery positive somewhere in the harness
EGR position sensor A internally shorted or pulling the circuit high
Broken or corroded ground return wire for the sensor
Faulty EGR valve or worn position sensor inside the valve assembly
PCM internal circuit failure or software calibration gap
Malfunctioning EGR solenoid or associated control wiring
Cracked, disconnected, or leaking vacuum hoses in the EGR control circuit
Heavy carbon deposits preventing the EGR valve from seating or opening correctly

How to Diagnose P0406

Connect an OBD2 scanner, confirm P0406 is stored, and record the freeze frame data—these conditions (RPM, load, temperature, speed) are what you will recreate during your test drive after repairs.
Scan for any additional diagnostic trouble codes; related failures often share a common root cause such as a wiring issue or vacuum leak. Check manufacturer TSBs for your specific make, model, and model year—many P0xxx codes have documented software updates or hardware fixes.
Perform a hands-on inspection of the entire EGR wiring harness and connectors. Look for frayed insulation, broken strands, corrosion at the terminals, and any signs of heat damage or rubbing against metal edges.
With the EGR position sensor disconnected, probe the harness-side signal terminal with a multimeter. If voltage reads high with the sensor unplugged, the harness itself has a short to power. If the reading drops to normal reference voltage, the sensor is the failed component.
Use a scanner to command the EGR valve open and closed while observing the position feedback and idle quality. Lack of response or erratic feedback narrows the problem to the valve, the solenoid, or vacuum actuation.
Remove the EGR valve and physically inspect the valve disk and intake passages for carbon buildup. Blocked passages prevent proper flow and can mask electrical faults.
Clear the code, drive the vehicle under the conditions recorded in the freeze frame, and verify P0406 does not reappear before considering the repair complete.

Repair Options for P0406

Tackle the most affordable possibilities before committing to expensive components:

Repair or replace vacuum hoses and damaged wiring: $80–$250
Clean the EGR valve and intake passages: $120–$350
Replace the EGR valve assembly: $250–$650

How Much Does Fixing P0406 Cost?

Your total bill depends entirely on what component is actually at fault. Parts and labor estimates below reflect typical independent shop rates in USD:

Wiring or vacuum line repair: $80–$250
EGR valve and passage cleaning: $120–$350
EGR valve replacement: $250–$650

Most P0406 repairs land somewhere between $80 and $650. A methodical diagnosis before purchasing parts keeps costs down—the full valve replacement is rarely the first thing to try.

Can You Drive with P0406?

Short-term driving is generally safe. You may notice rough idle, pinging under load, and increased emissions output. Some vehicles derate power or enter limp-home mode until the fault is cleared. If the engine stalls repeatedly or runs severely rough, treat it as a priority and have it diagnosed promptly.