Crankshaft Position Sensor Fault: What Diesel Drivers Need to Know
The crankshaft position sensor is a component most drivers never think about — until it fails. When it does, symptoms range from poor starting and rough running to a complete no-start. This guide explains what it does, how to recognise when it’s failing, and what professional diagnosis and repair involves.
In This Guide
- What the crankshaft position sensor does
- Common symptoms of a crankshaft sensor fault
- Fault codes P0335 to P0339 explained
- Why diesel engines are particularly affected
- Can you drive with a crankshaft sensor fault?
- What diagnosis and repair involves
- Frequently asked questions
What the Crankshaft Position Sensor Does
The crankshaft position sensor (CKP sensor, or crank sensor) monitors the rotational position and speed of the crankshaft in real time. It feeds a continuous signal to the engine control unit, which uses that data to determine precisely when to fire the fuel injectors.
On diesel engines, injection timing is critical. Diesel combustion relies entirely on compression ignition — there is no spark plug to trigger the event. The ECU’s injection timing decisions depend heavily on the crank sensor signal. Without it, or with a corrupted signal, injection timing breaks down and the engine either runs badly or refuses to start.
The sensor itself typically sits near the bottom of the engine block, close to the crankshaft pulley or the flywheel. It’s a relatively simple magnetic or Hall-effect sensor, but because of its position and the heat cycles it endures, it degrades over time.
Common Symptoms of a Crankshaft Position Sensor Fault
Symptoms vary depending on whether the fault is intermittent or complete. The most common signs include:
Engine cranks but won’t start
If the ECU can’t receive a valid crank signal, it may refuse to allow fuel injection. The engine turns over on the starter motor but won’t fire. This can happen suddenly or develop gradually as the fault worsens.
Intermittent stalling
A failing sensor that loses signal mid-journey can cut the engine without warning. This is particularly common when the engine is hot — heat causes the sensor’s internals to expand, which can open an already-developing crack at exactly the wrong moment.
Rough idle or diesel knock
If the sensor is sending a weak or erratic signal rather than failing completely, the ECU struggles to maintain accurate injection timing. This results in rough running, uneven idle, or a diesel knock that wasn’t there before.
Engine management light
A crank sensor fault almost always triggers the engine management light and stores one or more fault codes. The light alone doesn’t identify the specific component, but a professional diagnostic scan will retrieve the relevant codes quickly.
Hesitation under acceleration
With injection timing degraded, power delivery becomes uneven. Some drivers describe a flat spot during acceleration or a surging feeling — the engine builds power unevenly rather than pulling cleanly through the rev range.
Difficult hot restart
One of the clearest signs of thermal crank sensor failure: the engine starts fine when cold, runs normally until hot, then refuses to restart after a brief stop. Leave it 30 to 45 minutes to cool and it starts again. This pattern is very specific to heat-related sensor degradation.
Fault Codes P0335 to P0339 Explained
A diagnostic scan typically retrieves one or more of these OBD-II codes when the crankshaft position sensor is at fault:
| Fault Code | Description |
|---|---|
| P0335 | Crankshaft position sensor A — circuit malfunction |
| P0336 | Crankshaft position sensor A — range/performance issue |
| P0337 | Crankshaft position sensor A — circuit low input |
| P0338 | Crankshaft position sensor A — circuit high input |
| P0339 | Crankshaft position sensor A — circuit intermittent |
Manufacturer-specific codes often appear alongside these generic codes. Professional diagnostic equipment reads both, which matters on VAG, Mercedes, and Ford diesel platforms where live data parameters help confirm whether the sensor, the wiring, the connector, or the reluctor ring (the toothed wheel the sensor reads) is the actual failure point.
Why Diesel Engines Are Particularly Affected
On a petrol engine, a failing crank sensor causes misfires and poor starting, but the engine management system can often compensate to some degree using other sensor inputs.
On a diesel engine, injection timing precision is more critical. Without a reliable crank signal, injection either doesn’t happen correctly or doesn’t happen at all. This means diesel engines are far less forgiving of crank sensor issues. Where a petrol might run rough with a failing sensor, a diesel is more likely to stall completely, fail to restart, or run in a noticeably limited state.
Common rail diesel engines are particularly sensitive because injection events happen multiple times per combustion cycle at very high pressures, requiring tighter timing windows than older mechanical injection systems. The ECU depends on an accurate, continuous crank signal to manage this precisely.
Can You Drive With a Crankshaft Position Sensor Fault?
If the fault is intermittent and the vehicle is currently running, short low-risk journeys may be possible. But this isn’t something to rely on. The risk is the sensor fails completely mid-journey, cutting the engine without warning — a safety concern on faster roads.
If the engine has already stalled and won’t restart, or you’re experiencing repeated stalling, the vehicle needs diagnosing before driving further. A failing crank sensor tends to deteriorate rather than improve.
If the engine management light is on alongside any of the symptoms above, get a diagnostic scan as a priority. The scan confirms whether the crank sensor is the source or whether wiring, connector corrosion, or the reluctor ring is the actual issue.
Diagnosis and Repair: What to Expect
Step 1: Diagnostic scan with live data
A scan retrieves stored fault codes and — on professional-grade equipment — allows live monitoring of the crank sensor signal in real time. This is important because a code confirms the circuit has flagged a problem but doesn’t always confirm the sensor is the failed component. Wiring problems, a corroded connector, or a damaged reluctor ring can all produce identical fault codes.
Step 2: Physical inspection
The sensor and its wiring loom should be checked for damage, corrosion, and poor connections. The reluctor ring should be inspected for missing or damaged teeth — even a single missing tooth is enough to corrupt the signal pattern and cause symptoms identical to sensor failure.
Step 3: Sensor replacement and calibration
If the sensor is confirmed as the fault, replacement is usually straightforward. Some vehicles require the replacement sensor to be calibrated or coded to the ECU after fitting — especially common on newer common rail diesel platforms where professional-grade equipment is needed for the calibration step.
At Stoke Tuning, our advanced diagnostics include full live data analysis to confirm faults before parts are ordered and to verify correct output after repair.
Crankshaft Sensor Diagnosis in Stoke-on-Trent
Engine management light on? Stalling, rough running, or struggling to restart when hot? Stoke Tuning’s professional diagnostic service gives you accurate fault identification rather than guesswork.
Frequently Asked Questions
How much does a crankshaft sensor cost to replace?
The sensor itself is usually a modest part cost. Labour depends on how accessible it is on your specific vehicle. A proper diagnostic scan before replacement avoids unnecessary parts costs if the sensor turns out not to be the actual failure point.
Can a crankshaft sensor cause a complete no-start without prior warning?
Yes. In some cases the sensor fails suddenly rather than progressively. The first symptom is the engine cranking but not firing. This is more common when heat has been weakening the sensor over time and a demanding run finally tips it into complete failure.
Can I clear the fault code and keep driving?
Clearing the code removes the stored fault but doesn’t fix the underlying issue. If the sensor is genuinely faulty, the code will return and the symptoms will continue or worsen. Code clearing is useful as a verification step after a repair has been completed — not as a solution in itself.
Is a crankshaft sensor fault the same as a camshaft sensor fault?
They’re different sensors with different roles, though they work together. Camshaft position sensor faults (P0340 and related codes) affect injection phasing on modern multi-injection diesel systems. Both can cause similar running symptoms, which is why live data analysis during diagnosis is more reliable than fault codes alone.
Does a remap affect how the crankshaft sensor works?
No. A remap modifies the parameters stored within the ECU but doesn’t change how hardware sensors function. A diagnostic health check before remapping will identify any existing sensor issues that could affect how the remapped engine performs.
