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P040C – Exhaust Gas Recirculation Temperature Sensor A Circuit Low (OBD2)

P040C means "Exhaust Gas Recirculation Temperature Sensor A Circuit Low". The voltage the control unit measures in the exhaust gas recirculation (EGR) system circuit is below the permitted minimum. For a sensor this usually means the signal wire is shorted to ground or the reference voltage is missing; for an actuator, that its control wire is shorted to ground.

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P040C quick summary

Fault codeP040C
MeaningExhaust Gas Recirculation Temperature Sensor A Circuit Low
SystemPowertrain (Engine & Transmission)
VehiclesGeneric OBD2
SeverityMedium
Can I drive?Usually yes, but power may drop and the DPF fills faster on diesels. A valve stuck open can make the engine stall at idle – have it checked soon.
Repair cost€30 – €1,200

How the EGR system works

The EGR system feeds a controlled amount of exhaust gas back into the intake to lower combustion temperatures and nitrogen oxide (NOx) emissions.

An EGR valve between the exhaust and intake opens to let exhaust gas flow back into the engine. Modern valves are electric and have a position sensor; older ones are vacuum-operated and controlled by a solenoid. Diesels usually also have an EGR cooler, and some have a separate EGR throttle that creates the pressure difference needed for recirculation.

The control unit checks the EGR system by comparing the commanded valve position with the actual position, and by watching how the intake pressure or air mass changes when the valve opens. Codes report too little flow, too much flow, electrical faults in the valve or its position sensor, and faults of the EGR throttle.

Soot is the EGR system's enemy: on diesels and direct-injection engines the valve and passages clog with carbon, especially on cars used for short trips. A valve stuck open causes rough idle and stalling; one stuck closed raises NOx and can cause knocking on petrol engines.

Symptoms of P040C

  • Check Engine light on
  • Loss of power or limp mode
  • Rough idle, stalling (valve stuck open)
  • Black smoke and more frequent DPF regenerations (diesel)
  • Pinging/knocking under load (petrol, valve stuck closed)

Most common causes of P040C (most likely first)

Mechanic's Pro Tip

Cleaning just the EGR valve and leaving the sooted intake manifold behind gives only short-term relief. Check how much carbon is in the intake – on diesels the intake ports can be half blocked – and fix the reason for the excess soot (short trips, leaking injectors, a cold-running engine).

  1. 1.Signal or control wire shorted to ground

    Most likely

    Look for chafed insulation where the harness touches the engine, exhaust brackets or body edges.

  2. 2.Missing 5 V reference or supply voltage

    An open reference wire or a shorted sensor elsewhere on the same reference circuit can pull it down.

  3. 3.Internal short in the exhaust gas recirculation (EGR) system

  4. 4.EGR valve stuck with soot

    The most common cause, especially with lots of short trips.

  5. 5.Clogged EGR cooler or passages

    Restricts flow even when the valve opens fully.

  6. 6.Faulty EGR valve motor or position sensor

    Electric valves fail internally.

  7. 7.Vacuum supply or control solenoid fault (vacuum valves)

    Cracked hoses or a failed solenoid.

Step-by-step diagnostic guide

  1. 1

    Read codes and EGR data

    Read all codes and compare commanded and actual EGR position in live data at warm idle.

  2. 2

    Measure the signal voltage

    With the ignition on and the exhaust gas recirculation (EGR) system plugged in, back-probe the signal wire. Then unplug it and check the reference/supply and ground wires separately.

    Multimeter: Signal near 0 V = short to ground or no reference. Reference wire: about 5 V with ignition on. Signal wire to ground with connector unplugged: no continuity.

  3. 3

    Run the EGR output test

    Command the valve open and closed with the scan tool. It should move quickly and smoothly to both end positions; at idle, opening it should make the engine run rougher.

  4. 4

    Check vacuum and wiring

    On vacuum valves, check hoses and the control solenoid; on electric valves, check supply, ground and position sensor signal.

    Multimeter: Position sensor: typically 5 V reference and a signal that changes smoothly between about 0.5 and 4.5 V.

  5. 5

    Remove and inspect

    If the valve sticks, remove it and inspect the valve, the EGR cooler and the intake for carbon.

  6. 6

    Clean or replace and adapt

    Clean or replace the valve, clean the passages and run the EGR adaptation (basic setting) with the scan tool.

How do I read fault codes? →Where the OBD2 port is, which scanner to use and how to read and clear codes step by step.

Frequently asked questions

Can I keep driving with P040C?

Usually yes, but power may drop and the DPF fills faster on diesels. A valve stuck open can make the engine stall at idle – have it checked soon.

How much does it cost to fix P040C?

Typical repair costs are 30 € – 1200 €, depending on the cause and the vehicle. The most common fixes are: signal or control wire shorted to ground; missing 5 V reference or supply voltage; internal short in the exhaust gas recirculation (EGR) system. Always confirm the cause with the tests above before buying parts.

Can I clean an EGR valve instead of replacing it?

Yes, if the motor and sensor are fine and it only sticks because of soot. Afterwards, run the EGR adaptation. If the motor or sensor is faulty, replace it.

Is it legal to remove or disable the EGR?

No. In the EU it is illegal on road vehicles, it increases NOx emissions and the car can fail the technical inspection.

What is the difference between P040C and similar codes?

P040C belongs to a group of codes for the same component that differ in the type or location of the fault: P0409 – Exhaust Gas Recirculation Sensor A Circuit; P040A – Exhaust Gas Recirculation Temperature Sensor A Circuit; P040B – Exhaust Gas Recirculation Temperature Sensor A Circuit Range/Performance; P040D – Exhaust Gas Recirculation Temperature Sensor A Circuit High; P040E – Exhaust Gas Recirculation Temperature Sensor A Circuit Intermittent; P042E – Exhaust Gas Recirculation 'A' Control Stuck Open. The tests on this page apply to the whole group; the type of fault tells you where to start.

Related fault codes

This guide is for information only. Working on vehicles can be dangerous — if you are unsure, have the car checked by a qualified mechanic. Values can differ between model years; always check the manufacturer's repair information.