P0339
PowertrainCrankshaft Position Sensor B Circuit Intermittent
The crankshaft position sensor tells the ECU exactly where the crank is and how fast it's turning, and the engine uses that to fire the injectors and the spark at the right moment. The 'B' circuit here is a secondary sensor on engines that run more than one. P0339 specifically means the signal keeps cutting out or going erratic rather than failing outright, which is why this code is such a pain. The engine might run fine for a week, then stall at a junction for no obvious reason.
ⓘ Information only. This page provides general educational information about fault code P0339. We do not assess the urgency or safety implications of any specific fault. That requires in-person diagnosis by a qualified mechanic. Full terms.
Recommended next steps
Whether a fault is urgent, drivable, or routine depends entirely on the cause on a specific vehicle, and that can only be determined by a qualified mechanic with diagnostic equipment. If a warning light is illuminated, the most reliable next step is professional diagnosis.
What does P0339 mean?
P0339 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Crankshaft Position Sensor B Circuit Intermittent.
This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms commonly associated with this code
Symptoms that drivers often report alongside this code. Not all may apply to every case:
- • Engine cranks but won't catch on some attempts, then fires perfectly the next time
- • Stalling out of nowhere, often at idle or just as you slow for a roundabout
- • Lumpy idle when it does run, sometimes with the rev needle dancing
- • Engine warning light, which may flash under hard acceleration
- • Cranking that goes on longer than usual before the engine wakes up
- • Brief power loss or a flat spot under load that clears itself
Possible causes
Causes commonly associated with P0339, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. A worn sensor with internal coil breakdown, the usual culprit. They start out fine when cold and drop the signal once everything heats up, which is exactly why the fault is intermittent
- 2. Corroded or loose connector at the sensor plug. Worth checking before you condemn anything, since a dirty pin mimics a dead sensor
- 3. Chafed or oil-soaked wiring in the harness between the sensor and ECU. Heat and engine oil cook the insulation over the years
- 4. Poor earth or a flaky connection somewhere in the circuit, which gives you those random voltage dropouts
- 5. Reluctor ring trouble, either debris on the teeth or too big an air gap between sensor and ring
- 6. Coolant or oil getting into the wiring and breaking down the insulation from the inside
- 7. ECU input circuitry on its way out, but this is rare and you check it last
How mechanics typically diagnose
A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training.
- 1. Pull the freeze frame data alongside the code so you can see what the engine was doing when it logged. Was it hot? Idling? Under load? That points you straight at heat-related sensor failure
- 2. Unplug the sensor connector and look hard at it. Bent pins, green corrosion, oil sitting in the plug, any of these will throw an intermittent signal
- 3. Follow the harness from sensor to ECU and check for rubbing, melted insulation, or anywhere fluid could have got in. Wiggle it while watching live data if you can
- 4. Test the sensor's resistance against the manufacturer's figure, commonly somewhere around 0.5 to 2 kohm for the magnetic type. Out of spec means it's done
- 5. Watch the live signal while cranking and at idle, and look for the trace dropping out or going jagged. A clean sensor gives a steady, even pattern
- 6. Only once the wiring and sensor check out should you start questioning the ECU input circuit
Common questions about P0339
How do I work out whether it's the sensor or the wiring on my car? +
Use when the fault shows up as your clue. If P0339 only appears once the engine is hot or after a long run, the sensor itself is almost always the answer, because the coil inside breaks down as it warms. If it comes and goes over bumps or under vibration, you're looking at the wiring or connector instead. The cheapest move is to unplug the connector, check for corrosion and oil, give it a clean, and reconnect it firmly. Then drive it. If a clean connector cures it, you've saved yourself a sensor. If it keeps logging hot, replace the sensor.
Can I change the sensor myself or is that a garage job? +
On plenty of engines it's a single bolt and a connector, and the part runs roughly £30 to £90 for an aftermarket unit. The catch is access. The crank sensor often lives behind the engine, low down near the bellhousing or by the crank pulley, and you may need the car up on stands or even a few bits moved out of the way to reach it. If you can get to it comfortably, it's a sensible DIY job. If it's buried, an independent garage will swap it in under an hour of labour. Buy a known brand. The cheap unbranded sensors are notorious for failing again within months.
If I just clear the code, will it stay gone? +
For a few miles, maybe. But an intermittent fault doesn't fix itself, so the code comes back as soon as the sensor drops out again, usually within a drive cycle or two. The bigger issue is safety. Because this fault can stall the engine without warning, clearing it and carrying on means you might lose power on a busy junction or in motorway traffic. Sort out what's actually causing the dropout rather than wiping the memory and hoping.
Information only, not professional advice
The information on this page is provided for general guidance and educational purposes only. It is not a substitute for diagnosis or repair advice from a qualified mechanic. Always verify any fault before paying for repairs. carfaultcodes.co.uk accepts no liability for decisions made based on this information. Full terms →