P040F
Exhaust Gas Recirculation Temperature Sensor "A"/"B" CorrelationP040F is a generic OBD-II powertrain diagnostic trouble code: Exhaust Gas Recirculation Temperature Sensor "A"/"B" Correlation. It is logged by the engine control unit when the egr monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.
What P040F means
P040F is a generic powertrain code meaning the engine control module compared the readings from the two exhaust gas recirculation (EGR) temperature sensors, "A" and "B", and found they do not agree the way the calibration expects. Engines fitted with two of these sensors normally place them at different points in the EGR path, commonly one upstream and one downstream of the EGR cooler, so the ECU can work out how much heat the cooler is actually removing and control EGR flow accordingly.
This is a rationality or plausibility check rather than a simple circuit test. Under conditions where the module knows what the relationship between the two readings should be, such as after a long soak when both sensors ought to read close to ambient, or during steady cruise with a known EGR flow, it compares them. If the difference stays outside the allowed window for long enough, P040F is stored. Importantly, either sensor could be the wrong one, and both may in fact be reporting correctly while the physical system misbehaves.
Because of that, the fault splits into two families: an electrical problem at one sensor or its wiring, and a mechanical problem such as a restricted EGR passage, a failing EGR cooler, an exhaust leak, or heavy soot on one sensor tip. The code belongs to the EGR and emissions control system.
Common causes
Most-frequently reported root causes when P040F is logged.
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1
One of the two EGR temperature sensors has drifted or failed while the other still reads correctly.
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2
Heavy soot or oil build-up on one sensor tip has slowed its response so it lags the other.
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3
The connector at one of the sensors is corroded or water damaged.
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4
Signal wiring is chafed or heat damaged, or the shared sensor ground is poor.
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5
An EGR passage or cooler is blocked or partly restricted, so the gas temperature at one sensor is genuinely wrong.
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6
The EGR cooler is no longer cooling properly, often from internal fouling or a coolant flow problem.
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7
An exhaust or intake leak near one sensor is letting cooler air skew its reading.
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8
A previously fitted replacement sensor is the wrong specification for the engine.
Symptoms drivers notice
How to diagnose P040F
A typical diagnostic flow when this code is present.
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1
Read all stored codes and freeze frames, and note whether individual sensor circuit codes are stored alongside P040F, because those should be repaired first.
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2
Use the manufacturer's wiring and location data to establish which physical sensor is "A" and which is "B" on this engine.
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3
With the engine fully cold after an overnight soak, compare both readings in live data; both should sit close to ambient temperature, and a clear gap identifies the sensor that is off.
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4
Warm the engine and watch both readings through idle, cruise, and light load, looking for one sensor that lags, sticks, or moves erratically.
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5
Inspect both sensors, their connectors, and the harness for soot, oil, coolant, corrosion, chafing, and pins that are bent, spread, or pushed out.
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6
Check the sensor circuits back to the ECU for continuity, shorts, and a sound shared ground, since a poor ground can shift one reading without affecting the other.
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7
If both sensors and their wiring check out, inspect the EGR valve, passages, and cooler for blockage, fouling, or restricted coolant flow, because the readings may be truthfully reporting a mechanical fault.
Related powertrain codes
- P0400 — Exhaust Gas Recirculation Flow Malfunction
- P0401 — Exhaust Gas Recirculation Flow Insufficient Detected
- P0402 — Exhaust Gas Recirculation Flow Excessive Detected
- P0403 — Exhaust Gas Recirculation Circuit Malfunction
- P0404 — Exhaust Gas Recirculation Circuit Range/Performance
- P0405 — Exhaust Gas Recirculation Sensor A Circuit Low
Frequently asked questions
Can I keep driving with P040F?
Generally yes for normal use, as the engine still runs. If it drops into reduced power, smokes heavily, or the temperature gauge climbs, stop and have it inspected, since a failing EGR cooler can affect the cooling system.
Will it fail an emissions test?
Yes. It is an EGR system code, so the stored fault and the check engine light will fail an OBD-based inspection.
Does this mean I need two new sensors?
No. The code only says the two readings disagree, not that both sensors are bad. Comparing them on a fully cold engine usually shows which one is wrong, and quite often the real fault is wiring, soot, or a blocked EGR passage rather than a sensor at all.
Is this expensive to fix?
It varies widely. A connector repair or a single sensor is inexpensive, but if the cause is a fouled EGR cooler or a blocked EGR circuit, the labour to remove, clean, or replace those parts is the bulk of the bill.
Disabling P040F in software
RaceTune can permanently disable P040F — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.
ECU families we can disable P040F on
We hold the DaVinci A2L disable definitions for these families, so the exact P040F path and mask addresses are mapped. verified marks a confirmed disable definition. We support many more — upload your file and our identifier will match it automatically.
- Bosch MD1CP004 verified
Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.
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