What does a crankshaft position sensor do?

The crank sensor tells the ECU when the crankshaft reaches top dead centre (TDC) for cylinder 1. This signal is critical — the ECU uses it to time fuel injector pulses and spark plugs. Without this signal, the engine cannot fire correctly and will not start or run.

Can I drive with P0335?

No. Without a crank position signal, the ECU cannot control spark and fuel timing. The car will either not start, or start and stall immediately. Some older cars without electronic fuel injection might coast to a stop with limp-home capability, but modern cars will shut down.

Is the sensor expensive?

Crankshaft sensors cost £80–£200 for the part and £100–£250 in labour, totalling £180–£450. Compared to other engine repairs, this is relatively inexpensive. If the reluctor ring is damaged, costs can be higher (£500–£1500) because the crankshaft damper or pulley may need replacement.

Can the reluctor ring break?

Yes, but it's rare. The reluctor ring (the toothed target on the crank pulley or crankshaft) can be damaged by impact, corrosion, or wear. If damaged, the sensor cannot read crank position. Replacement requires removing the crank pulley, a more involved job.

Will this fault code cause my car to fail its MOT?

It depends on which warning light the code is triggering. Since 2018, any car presenting with an illuminated amber Engine Management Light (EML) at the MOT is a Major failure under DVSA rules — even if the car drives perfectly. A red warning light is always a Major or Dangerous failure depending on context. If clearing the fault makes the light go out and the code does not reappear during the pre-test drive, you will pass; if the code returns within minutes of clearing, the underlying fault must be fixed before MOT day. A tester is required to fail the car on the light being on, regardless of whether the underlying fault is something safety-critical or not. For codes that affect emissions specifically (catalyst, lambda, EGR), the car may also fail the actual emissions check. Fix the cause, clear the code, and drive the car for a few miles before the test.

P0335 — Crankshaft Position Sensor A Circuit

What Is P0335?

P0335 is a crankshaft position sensor circuit fault. The crankshaft sensor (CKP) generates a signal as the crankshaft rotates, sending a pulse to the ECU each time the crank passes a specific position (usually cylinder 1 TDC — top dead centre). The ECU uses this signal to synchronise fuel injection and ignition timing with engine speed and piston position. Without this signal, the ECU cannot fire the injectors or spark plugs and the engine will not start.

P0335 means the sensor is not sending any signal, is sending an intermittent signal, or the wiring is faulty. This is a critical fault — the car is non-functional.

Common Symptoms

Engine will not start or cranks but fails to start
Engine starts and runs, then stalls without warning at speed
Check engine light illuminated or flashing
No fuel pump sound when turning the ignition key
Complete loss of power or inconsistent running if it somehow starts

Common Causes

Crankshaft Sensor FailureThe sensor element has failed electrically, generating no signal or a signal too weak for the ECU to read.

Damaged Wiring HarnessThe sensor wiring is cut, corroded, or pinched, preventing signal transmission from the sensor to the ECU.

Loose or Corroded ConnectorThe sensor connector is not fully seated, or the pins are corroded, creating an open circuit.

Reluctor Ring DamageThe toothed target on the crankshaft pulley is bent, worn, or missing teeth, preventing the sensor from detecting crank rotation.

ECU Input Circuit FailureThe ECU's crank sensor input circuit is damaged or faulty, unable to receive the sensor signal even if the sensor is working.

Incorrect Sensor InstallationThe sensor gap (distance from reluctor ring) is too large, preventing the sensor from generating a strong enough signal.

How to Diagnose P0335

Inspect Sensor Connector

Locate the crankshaft sensor (usually mounted on the engine block near the crankshaft pulley, front of engine). Disconnect the connector and inspect for corrosion, bent pins, or water damage. Clean any corrosion with electrical contact cleaner. Reseat the connector firmly, clear the code, and test. A loose connector often fixes P0335.

Inspect Wiring and Harness

Follow the crank sensor wiring from the connector back to the ECU. Look for cuts, abrasions, heat damage, or pinched wires. Check for rodent damage. If the harness passes through engine bay clips, verify it hasn't rubbed through insulation. Repair or replace damaged wiring sections.

Monitor Sensor Signal on Scanner

Connect an OBD scanner and monitor the crankshaft position sensor signal in live data mode while cranking the engine. The signal should show a regular oscillating pattern. If the signal is frozen, erratic, or shows no activity, the sensor is likely faulty.

Check Reluctor Ring for Damage

Remove the crankshaft pulley or damper to access the reluctor ring (the toothed target). Inspect for bent, missing, or corroded teeth. If the ring is damaged, it must be replaced. This requires removing the pulley and is a professional-level task.

Test Sensor Resistance

Disconnect the sensor. Using a multimeter, measure resistance across the sensor terminals. Most crankshaft sensors should read 200–900 ohms depending on type. Infinite resistance or zero ohms indicates a failed sensor. Compare your reading to the manufacturer's specification.

Non-Starter P0335 is a critical fault. The car will not start without a crank signal. Check the connector and wiring first — free fixes. If the sensor is faulty, replacement is necessary. Do not attempt to "limp" the car — it won't run.

Mechanic's Corner — Crankshaft Sensor Faults

P0335 means the ECU can't get a clean signal from the crankshaft position sensor, and on many vehicles this will cause a complete no-start — the ECU won't fire the injectors or ignition without a valid crank signal. The sensor itself is relatively cheap (£15–£40), but before replacing it, check the reluctor ring on the crankshaft. This toothed ring can crack or shed teeth, particularly on high-mileage vehicles, and no sensor in the world will give a clean signal from a damaged ring.

The wiring is also worth checking — the CKP sensor sits low on the engine block in a stone-strike and oil-splash zone. I've diagnosed more P0335 faults caused by chafed wiring or a corroded connector than by an actually failed sensor.

Verdict

P0335 diagnosis begins with connector and wiring inspection — often the culprit and a free fix. If those are clean and tight, the sensor needs testing. A faulty sensor costs £180–£450 installed. Reluctor ring damage is rare but can require expensive crankshaft pulley replacement (£500–£1500). Start with the cheapest checks first.