What is the EGR position sensor?

It is a sensor integrated into the EGR valve assembly that tells the ECU the exact opening percentage of the valve. The ECU uses this feedback to fine-tune EGR flow for emissions control.

Can I clean the EGR valve myself?

Yes, if the valve is carbon-coated and the sensor is working correctly. Remove the valve, soak in EGR cleaner, and clean the disc and seat. However, a valve with a faulty integrated sensor needs replacing entirely.

How much does an EGR valve cost?

EGR valves with integrated position sensors typically cost £80–£300 depending on vehicle. Budget brands are available but OEM or quality aftermarket is recommended for reliability.

Will P0404 cause an MOT failure?

Yes — the engine management light illuminates, which fails the MOT. The EGR system is also checked directly on some vehicles during the emissions test.

Is P0404 the same as P0405?

Related but different. P0404 is a range/performance fault (position mismatch). P0405 is an EGR sensor circuit low (electrical fault in the sensor circuit). P0404 often points to mechanical causes; P0405 to electrical ones.

Can I blank the EGR and resolve P0404?

Blanking the EGR removes the mechanical cause but does not resolve the electrical monitoring circuit. The ECU will continue to detect the position sensor fault and log codes. Blanking alone does not fix P0404.

P0404 — EGR Circuit Range/Performance

What Is P0404?

P0404 is stored when the ECU detects that the EGR (exhaust gas recirculation) valve position sensor is giving a signal outside the expected range for the current operating conditions. The EGR valve uses a position sensor to feed back its exact opening percentage to the ECU — when the commanded position and actual position do not match, P0404 is logged.

This is often caused by a carbon-coated EGR valve that cannot move freely — the sensor detects a discrepancy between what the ECU commanded and what the valve actually achieved. It can also be caused by a failing position sensor or damaged wiring.

Common Symptoms

Engine management light on
Poor idle quality or rough running
Reduced low-rev torque
Increased fuel consumption
Black smoke on diesel
Failed emissions test
Possible limp mode

Common Causes

Carbon-seized EGR valve — cannot reach the commanded position, position sensor reports mismatch

Failed EGR position sensor — giving incorrect position feedback to the ECU

Damaged wiring to the EGR position sensor — open circuit or short

Corroded connector at the EGR valve assembly

Sticking EGR valve — partially blocked by carbon, moving sluggishly

ECU output fault — not commanding the correct valve position (rare)

How to Diagnose P0404

Check EGR Live Data

Connect a scanner with live data capability and monitor EGR valve commanded position versus actual position. If they diverge significantly, the valve is not reaching the commanded position — carbon seizure or a position sensor fault.

Inspect the EGR Valve

Remove and inspect the EGR valve. Carbon build-up is immediately obvious — the valve may be stuck partially open or closed. A clean valve with the disc moving freely suggests the sensor is the fault.

Test Position Sensor Wiring

Check the sensor connector for corrosion. Test reference voltage (5V) and measure signal wire continuity back to the ECU. An open circuit or corroded pin causes erratic position feedback.

Clean the EGR Valve

A carbon-coated valve can sometimes be cleaned with EGR cleaner fluid. However, valves with a failed position sensor integrated into the body need replacement of the complete assembly.

Replace the EGR Valve

On most modern vehicles, the EGR valve and position sensor are a single assembly and are replaced as a unit. Ensure the EGR cooler and ports are also cleaned during this job to prevent rapid re-fouling.

Verify with Live Data After Repair

After replacing the valve, check live data again to confirm commanded and actual positions match. The ECU may need a reset to re-learn the valve behaviour on some vehicles.

EGR Valve CleaningWhile cleaning a carbon-coated EGR valve can provide a temporary fix, a valve with a faulty integrated position sensor must be replaced. Attempting to clean a valve with an electrical fault will not resolve P0404.

Verdict

Check live position data first to confirm the valve is not reaching commanded positions. Carbon seizure is the most common cause — clean or replace the valve. Check wiring if the valve moves freely.