Common ECU Fault Codes & Diagnostics
A plain-English overview of the fault codes we see most often, covering DPF, AdBlue, EGR, turbo, fuel pressure and sensor systems. This guide is here to help you understand what a warning light might mean, not to replace a proper diagnostic check.
What are ECU fault codes?
Every modern vehicle's ECU continuously monitors sensors and actuators across the engine, emissions and transmission systems. When a reading falls outside its expected range, the ECU stores a diagnostic trouble code, commonly called a P-Code, and usually lights up the engine management light on the dashboard.
These codes often appear after a component wears out, a sensor drifts, or following modifications that change how the engine or emissions systems behave. A code tells you where the ECU noticed a problem. It doesn't, on its own, tell you why.
Fault code categories
DPF (Diesel Particulate Filter)
The DPF traps soot from diesel exhaust gases and periodically burns it off in a process called regeneration. When the filter blocks, regeneration fails, or a related sensor drifts out of range, the ECU logs a fault and often restricts engine performance.
| Code | System | General meaning |
|---|---|---|
| P2002 | DPF efficiency | Filter efficiency below the expected threshold, often linked to soot loading. |
| P2452 | DPF pressure sensor | A/B pressure signal correlation issue between the DPF's differential sensors. |
| P244A / P244B | DPF pressure, low/high | Differential pressure reading outside the expected range for current conditions. |
| P2463 | Soot accumulation | Excessive soot build-up detected, often after repeated short journeys. |
Examples only, not an exhaustive list. Different manufacturers use different code sets and diagnostic logic, so the same fault can appear under a different code depending on the vehicle.
AdBlue / SCR
Selective Catalytic Reduction (SCR) uses AdBlue (diesel exhaust fluid) to reduce NOx emissions. Faults here typically relate to fluid quality, injector dosing, tank sensors, or the SCR catalyst itself.
| Code | System | General meaning |
|---|---|---|
| P20EE | SCR NOx efficiency | Reductant system NOx conversion below the required threshold. |
| P204F | Reductant quality | AdBlue quality signal outside the expected concentration range. |
| P203F | Reductant level | AdBlue tank level sensor reporting an implausible or low reading. |
| P2201 | NOx sensor circuit | NOx sensor circuit signal outside normal operating range. |
Examples only, not an exhaustive list. Different manufacturers use different code sets and diagnostic logic, so the same fault can appear under a different code depending on the vehicle.
EGR (Exhaust Gas Recirculation)
The EGR valve recirculates a portion of exhaust gas back into the intake to lower combustion temperatures and reduce NOx. EGR faults are common on higher-mileage vehicles due to carbon build-up affecting valve movement.
| Code | System | General meaning |
|---|---|---|
| P0401 | EGR flow, insufficient | Detected exhaust gas flow lower than the ECU expects for current conditions. |
| P0402 | EGR flow, excessive | More exhaust gas recirculating than requested, often a stuck-open valve. |
| P0404 | EGR circuit range/performance | Valve position signal outside the expected operating range. |
| P0409 | EGR position sensor circuit | Electrical fault on the valve's position sensor circuit. |
Examples only, not an exhaustive list. Different manufacturers use different code sets and diagnostic logic, so the same fault can appear under a different code depending on the vehicle.
Turbo & Boost
Modern turbochargers rely on precise boost control through a wastegate or variable-vane actuator. Boost faults can stem from actuator wear, vacuum leaks, or the sensors that monitor manifold pressure.
| Code | System | General meaning |
|---|---|---|
| P0234 | Overboost condition | Boost pressure has exceeded the ECU's expected upper limit. |
| P0299 | Underboost condition | Boost pressure lower than expected, often from a leak or worn actuator. |
| P0046 | Turbo boost control circuit | Range or performance fault on the boost control solenoid circuit. |
| P2563 | Turbo vane position | Variable geometry turbo vane position sensor signal out of range. |
Examples only, not an exhaustive list. Different manufacturers use different code sets and diagnostic logic, so the same fault can appear under a different code depending on the vehicle.
Fuel Pressure
Fuel pressure faults cover both the low-pressure supply side and the high-pressure common-rail side on diesels and direct-injection petrols. These codes often point towards pump wear, regulator faults, or injector leak-back.
| Code | System | General meaning |
|---|---|---|
| P0087 | Fuel rail pressure, low | Rail pressure below the commanded value for current engine demand. |
| P0088 | Fuel rail pressure, high | Rail pressure above the commanded value, often a regulator issue. |
| P0191 | Fuel rail pressure sensor | Range or performance fault on the rail pressure sensor circuit. |
| P00C6 | Fuel pump control performance | Low-pressure pump not meeting the ECU's commanded output. |
Examples only, not an exhaustive list. Different manufacturers use different code sets and diagnostic logic, so the same fault can appear under a different code depending on the vehicle.
Intake & Swirl Flaps
Swirl flaps sit in the intake manifold and adjust airflow characteristics to improve combustion efficiency at lower engine speeds. They're prone to mechanical wear and carbon build-up over time.
| Code | System | General meaning |
|---|---|---|
| P2015 | Intake manifold runner position | Sensor B circuit range/performance fault on the runner position. |
| P2004 | Runner control, stuck open | Manifold runner control stuck in the open position. |
| P2009 | Runner control circuit, low | Low voltage fault on the runner control circuit. |
| P2070 | Runner position, low | Runner position sensor reading lower than expected. |
Examples only, not an exhaustive list. Different manufacturers use different code sets and diagnostic logic, so the same fault can appear under a different code depending on the vehicle.
Glow Plug System
Glow plugs preheat diesel combustion chambers for reliable cold starts and, on many engines, assist emissions control during warm-up. A single faulty plug or control module fault can trigger a warning light and rough starting.
| Code | System | General meaning |
|---|---|---|
| P0670 | Glow plug control circuit | General fault on the glow plug control module circuit. |
| P0671�P0678 | Individual cylinder circuits | Circuit fault isolated to a specific cylinder's glow plug. |
| P0380 | Glow plug circuit A | Malfunction detected in glow plug circuit bank A. |
Examples only, not an exhaustive list. Different manufacturers use different code sets and diagnostic logic, so the same fault can appear under a different code depending on the vehicle.
NOx Sensors
NOx sensors monitor nitrogen oxide levels before and after the SCR catalyst to confirm emissions reduction is working correctly. They are sensitive components and can drift or fail with age.
| Code | System | General meaning |
|---|---|---|
| P2201 | NOx sensor circuit | Signal outside the expected operating range. |
| P229F | NOx sensor correlation | Upstream and downstream sensor readings don't correlate as expected. |
| P0170 | Fuel trim / NOx bank 1 | Fuel trim fault that can present alongside NOx sensor irregularities. |
Examples only, not an exhaustive list. Different manufacturers use different code sets and diagnostic logic, so the same fault can appear under a different code depending on the vehicle.
EGT Sensors
Exhaust Gas Temperature sensors protect the turbo, DPF and SCR system by monitoring temperature at key points in the exhaust. A faulty reading can affect DPF regeneration and turbo protection strategies.
| Code | System | General meaning |
|---|---|---|
| P0545 | EGT sensor circuit, bank 1 | Low input fault on the exhaust gas temperature sensor circuit. |
| P0546 | EGT sensor circuit, high | High input fault on the same circuit. |
| P244E | EGT sensor correlation | Reported temperature doesn't correlate with expected values at that sensor position. |
Examples only, not an exhaustive list. Different manufacturers use different code sets and diagnostic logic, so the same fault can appear under a different code depending on the vehicle.
MAF / MAP Sensors
Mass Air Flow and Manifold Absolute Pressure sensors tell the ECU how much air is entering the engine, which underpins fuelling and boost calculations. A dirty or failing sensor can cause poor running and incorrect fuel trims.
| Code | System | General meaning |
|---|---|---|
| P0101 | MAF circuit range/performance | Airflow reading outside the expected range for current conditions. |
| P0102 | MAF circuit, low input | Signal lower than expected, often a wiring or sensor fault. |
| P0106 | MAP circuit range/performance | Manifold pressure reading outside expected parameters. |
| P0069 | MAP/barometric correlation | Manifold pressure doesn't correlate with barometric pressure as expected. |
Examples only, not an exhaustive list. Different manufacturers use different code sets and diagnostic logic, so the same fault can appear under a different code depending on the vehicle.
A code is a starting point, not a diagnosis
Fault codes alone don't diagnose a vehicle
- �A fault code points to a system, not necessarily the exact part at fault.
- �The same code can have several different underlying causes.
- �Fitting a new part based on a code alone can be a costly guess if the root cause is missed.
- �Professional diagnostics are recommended before replacing any components.
Professional services
If you've spotted one of these codes, or your engine management light is on, we can help. We diagnose the vehicle properly before recommending any solution.
FAQ
A P-Code is a standardised diagnostic trouble code stored by the ECU when it detects a fault, typically starting with the letter P for powertrain. It points to the general area of a problem, not the exact cause.
It depends on the code and severity. Some DPF faults are safe to drive with in the short term, while others trigger limp mode or risk further damage. If in doubt, get it checked before continuing to drive.
No. Clearing a code only resets the warning light. If the underlying fault is still present, the code will return, often alongside a fresh regeneration or driving cycle.
The ECU has detected a reading outside its expected range somewhere in the engine, emissions or transmission systems. It could be minor or serious, which is exactly why a proper diagnostic check matters.
Generic P-Codes are shared across manufacturers, but many brands also use manufacturer-specific codes and their own diagnostic logic. The same generic code can mean slightly different things from one platform to another.
Talk to us before you guess
Every vehicle is different. We diagnose each one individually before recommending the correct solution, rather than assuming a code means the same thing twice. If your engine management light is on, get in touch before ordering parts or attempting to diagnose or modify the vehicle yourself.