Why does my car only fail to start when hot?

The crankshaft sensor is close to the exhaust and the engine block, both of which get very hot. Internal sensor components expand and fail electrically when hot, then recover as the engine cools. This classic hot-start failure pattern is almost diagnostic of a failing CKP sensor.

Can I replace the crankshaft sensor myself?

On many engines yes — the sensor is accessible with the car on ramps or axle stands. Verify the location for your engine code before starting, as some sensors are buried behind the crankshaft pulley and require pulley removal.

Does the crankshaft sensor need programming?

No — it is a direct replacement on all common applications. On some Ford and GM engines a crankshaft position relearn procedure is recommended after replacement to allow the ECU to relearn reluctor ring variation.

How long does a crankshaft sensor last?

Most sensors last 100,000 miles or more but are susceptible to heat damage and oil contamination from a leaking front crankshaft seal. Address any oil leak at the same time.

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Crankshaft Sensor buying guide — what to know before you buy

The crankshaft sensor — also called the crankshaft position sensor, crank sensor, CKP sensor, or engine speed sensor — reads the position and rotational speed of the crankshaft from a reluctor ring, providing the engine control unit with the data it needs to control ignition timing and fuel injection sequence. Without this signal the engine cannot start. Intermittent failure — common on hot engines — causes stalling, hard hot-starts, and random misfires. CarParts247 stocks OEM and aftermarket crankshaft sensors from Bosch, Continental, and Hella for a vast range of petrol and diesel engines across all major manufacturers.

Symptoms of a failing crankshaft sensor

Because the ECU depends on the crank sensor for ignition and injection timing, failure produces some of the most dramatic drivability symptoms of any single component.

Engine cranks but will not start — no crank signal means no spark or injection
Intermittent stalling, particularly after the engine reaches operating temperature
Hard hot-start: engine starts fine when cold but refuses to start when warm
Random misfires and hesitation at all engine speeds
DTC codes P0335 to P0339 (crankshaft position sensor circuit)

Replacement procedure

The crankshaft sensor is typically mounted at the bottom of the engine block near the reluctor ring, behind the crankshaft pulley, or at the transmission bellhousing. Access varies considerably. Remove the single retaining bolt, disconnect the wiring connector, and withdraw the sensor. Note the air gap specification — some applications require a shim or specific clearance between the sensor tip and the reluctor ring.

Check the reluctor ring for damaged or missing teeth before refitting — a damaged ring will cause the same fault codes as a failed sensor.

Testing

A Hall-effect sensor requires supply voltage, earth, and should output a square wave during cranking. A variable-reluctance (VR) sensor produces an AC sine wave whose amplitude increases with engine speed. An oscilloscope captures both waveform types and makes tooth damage on the reluctor ring immediately visible as a distorted waveform.