P0315
PowertrainCrankshaft Position System Variation Not Learned
Most of the time this turns up after someone has changed the crankshaft sensor, swapped or reflashed the ECU, or just disconnected the battery, and the relearn procedure was never run afterwards. The ECU stores a tiny variation map of how the crank reluctor wheel spins so it can pinpoint misfires accurately. Until it relearns that pattern, it flags P0315 to say it doesn't trust its own timing reference yet. It can also point to a tired sensor or wiring fault, but the missed relearn is the usual story.
ⓘ Information only. This page provides general educational information about fault code P0315. 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 P0315 mean?
P0315 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Crankshaft Position System Variation Not Learned.
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 warning light on, sometimes with no other obvious change in how the car drives
- • Rough or lumpy idle, especially in the first few seconds after starting
- • Hesitation or a flat spot when you put your foot down
- • Harder to start, more noticeable when the engine is already warm
- • The odd stall just after start-up before it settles
- • Misfire detection going haywire and throwing random cylinder codes alongside this one
Possible causes
Causes commonly associated with P0315, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Relearn procedure never carried out, by far the most common reason. Happens after sensor replacement, an ECU swap or reflash, or simply pulling the battery
- 2. Failing crankshaft position sensor giving a weak or noisy signal the ECU can't lock onto
- 3. Corroded, chafed, or loose wiring and connectors in the crank sensor circuit, often near the gearbox bellhousing where it cooks
- 4. Weak or flat battery that drops voltage too low for the relearn routine to finish cleanly
- 5. Worn timing chain or a damaged reluctor ring throwing the variation pattern off, common on higher-mileage chain-driven engines
- 6. Out-of-date or corrupted ECU software that needs a calibration update from the manufacturer
- 7. Faulty ECU, which is rare and should be the last thing you blame
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 all stored codes and check the freeze-frame data. If P0315 landed right after battery, sensor, or ECU work, you've probably already found your answer
- 2. Look for company it's keeping. Codes like P0335, P0339, or P0340 alongside it usually mean a real sensor or wiring problem rather than just a missed relearn
- 3. Check the battery and charging system first. You want roughly 12.4 to 12.6V with the engine off and 13.7 to 14.7V running. A tired battery will sabotage the relearn every time
- 4. Inspect the crank sensor connector and harness for green corrosion, split insulation, or backed-out pins. Wiggle the loom while watching live data if you can
- 5. Run the manufacturer-specific crankshaft variation relearn with a bidirectional scan tool. This is the actual fix in most cases and takes a few minutes once you know the routine
- 6. If the relearn refuses to complete, back-probe the sensor for signal and resistance, then look harder at the timing chain stretch or reluctor wheel
Common questions about P0315
How worried should I be about driving with this showing? +
For short hops it's usually fine, and plenty of cars drive normally with P0315 set after a battery change. The catch is that the ECU can't trust its misfire detection until it relearns, so it might mask or mimic a genuine misfire. If the car is stalling, running rough, or being awkward to start, get the relearn done before you rely on it for anything important. A quick scan-tool session sorts most cases.
Is this the sensor itself or just a wiring problem? +
Often it's neither. The most common trigger is simply that the relearn was never run after work was done, so the parts are perfectly healthy. When there is a hardware fault, it's more likely to be corroded connectors or a chafed wire near the bellhousing than the sensor failing outright. Check the loom and the battery before you start throwing a new sensor at it, because a fresh sensor without a relearn will still leave the code there.
How long does the repair actually take? +
If it's just the relearn, you're looking at minutes once the tool is plugged in, often quicker than reading the codes in the first place. A sensor swap adds maybe half an hour to an hour depending on how buried it is, plus the relearn afterwards. Wiring repairs depend entirely on how far the damage runs. Timing chain or reluctor work is a different world and can tie a car up for the best part of a day.
Is a cheap aftermarket crank sensor worth it or should I stick to OEM? +
For the sensor, a decent branded aftermarket part from a name like Bosch, Delphi, or Febi is usually fine and a lot cheaper than dealer kit. Steer well clear of the bargain unbranded ones off auction sites, because a noisy or out-of-spec signal will keep P0315 alive and have you chasing your tail. If the car is something where timing precision matters, the few extra pounds for a known brand or genuine part pays for itself in not doing the job twice.
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 →