The p0335 code usually points to a crankshaft position sensor circuit fault, but the real job is figuring out whether the sensor, the wiring, the reluctor ring, or engine timing is actually to blame. This guide breaks down the symptoms, the diagnostic checks that matter, and the repair decisions that save time and money, especially if you are dealing with a no-start or stalling issue in a UK car.
What matters most before you replace anything
- P0335 means the engine control unit is not seeing a valid crankshaft position signal.
- Hard starting, stalling, and sudden cut-outs are the most common real-world symptoms.
- The sensor is only one possible fault; wiring, connector corrosion, a damaged tone ring, or timing issues can trigger the same code.
- A live-data scan while cranking is one of the fastest ways to narrow the fault down.
- In the UK, basic diagnostics often cost about £50-£100, while a straightforward sensor replacement is usually a few hundred pounds or less.
What the fault means in plain English
The crankshaft position sensor tells the engine control module where the crankshaft is and how fast it is moving. Without that information, the ECU cannot time fuel injection and ignition correctly, which is why this fault can quickly turn into a no-start or a stall. In other words, this is not just an emissions nuisance; it is a core engine-synchronisation problem.
Technically, the code is usually described as a crankshaft position sensor circuit malfunction. That wording matters, because it tells you the fault may be in the sensor itself, the signal path, the power supply, the ground, or the mechanical trigger wheel the sensor reads. I would treat it as a signal problem first and a parts problem second. Once that is clear, the symptoms make a lot more sense.
The symptoms that usually show up first
The most common clue is a warning light paired with poor starting behaviour. On some cars the engine will crank normally but never catch; on others it will start, run roughly, and then stall without much warning. Intermittent faults can be especially awkward because the car may behave normally for days and then fail the moment conditions change.
- Check engine light on with no obvious drivability issue yet.
- Cranks but will not start, especially if the ECU is not seeing engine speed during cranking.
- Starts and then stalls, often after heat soak or after a short drive.
- Rough idle or misfires, because ignition timing and fuel delivery lose a reliable reference.
- Sudden engine cut-out while driving, which can feel like the ignition was switched off.
- Tachometer drops to zero during cranking on some vehicles, which is a useful clue rather than a guarantee.
One thing I see drivers underestimate is how quickly a crank sensor fault can move from “annoying” to “stranded”. If the symptom pattern is already pointing toward signal loss, the next step is to work out why the signal disappeared in the first place.
Why it happens more often than a dead sensor
A failed sensor is common, but it is not the only realistic explanation. In practice, I would put wiring damage and connector problems very high on the list, especially on cars that have lived through heat, road salt, oil leaks, or previous repairs around the engine bay. The sensor itself may be fine while the signal gets lost before it reaches the ECU.
| Likely cause | Why it triggers the fault | Clue I would look for |
|---|---|---|
| Failed crankshaft position sensor | The sensor stops generating a usable signal | Intermittent or complete no-start, often worse when hot |
| Damaged wiring or corroded connector | The ECU loses power, ground, or signal continuity | Fault appears after vibration, rain, or engine movement |
| Reluctor ring or tone wheel damage | The sensor cannot read crank position correctly | Missing tooth, debris, or visible mechanical damage |
| Low battery voltage or slow cranking | Signal amplitude can fall too low for the ECU to trust | Weak battery, slow starter, cold-weather complaints |
| Timing belt or chain issue | Mechanical timing and crank signal no longer line up properly | Related camshaft codes, rough running, recent timing work |
| ECU or software fault | The module misreads or misprocesses the signal | Rare, but worth considering after all physical checks pass |
The important takeaway is simple: a stored fault code is not the same thing as a failed part. That distinction is where a lot of money gets wasted, and it leads straight into the diagnostic process.
How I would diagnose it step by step
I would start with the fastest, least invasive checks and only move to parts replacement once the data points in one direction. On a crankshaft sensor fault, live data and basic electrical testing usually tell you more than guesswork ever will. That is especially true if the vehicle is fitted with a Hall-effect sensor, which uses power, ground, and a switching signal, versus an inductive sensor that generates its own AC signal while the engine turns.
- Check battery condition and starter speed first. A weak battery can distort the test results and make a good sensor look bad.
- Read all stored and pending codes, not just the main one. Related P0336-P0339, camshaft, or misfire codes often change the diagnosis.
- Look at live RPM while cranking. If the scan tool shows zero engine speed, the ECU is not receiving a believable crank signal.
- Inspect the sensor, connector, and harness for oil contamination, corrosion, chafing, broken clips, or heat damage.
- Test power, ground, and signal with the correct method for the sensor type. A multimeter can help, but an oscilloscope is much better when the fault is intermittent.
- If the signal looks unstable, check the reluctor ring or tone wheel and confirm the mechanical timing is still correct.
Freeze-frame data is worth reading here as well. It shows the engine conditions when the fault set, such as temperature, RPM, and battery voltage. That snapshot often tells me whether the fault appears during cranking, at idle, or after the engine warms up. From there, the pattern usually becomes much easier to trust.
Whether you should keep driving or book the garage now
If the engine is stalling, misfiring heavily, or refusing to start, I would not keep driving it. That is not a code you want to ignore on a busy road, because the failure can happen without much warning. If the car starts and runs normally but the fault is stored, you may be able to make a short, careful trip to a garage, but I would avoid long journeys and motorways until the signal problem is understood.
The practical rule is this: if the engine speed signal is unstable, the car can become unpredictable. A crank sensor fault can also stop the fuel and ignition strategy from working properly, so the issue is about more than convenience. Once reliability is in doubt, the next question is usually cost, which is where many owners need a realistic expectation rather than a guess.
What the repair usually involves and what it costs in the UK
For a straightforward fault, the fix may be as simple as replacing the sensor and clearing the code. In other cases, the repair is really about the wiring, connector, or mechanical trigger wheel, so the cost rises because the diagnosis takes longer than the part change itself. RAC currently lists a vehicle diagnostic at £99, while ClickMechanic puts a typical crankshaft sensor replacement at about £150, with a range of £120-£300.
| Job | Typical UK cost | What it usually covers |
|---|---|---|
| Basic diagnostic check | £50-£100 | Fault code read, live-data check, and a quick inspection |
| Main dealer diagnostic | £100-£200+ | Brand-specific software and deeper module testing |
| Crankshaft position sensor replacement | £120-£300 | Accessible sensor swap with standard labour |
| Wiring or connector repair | £60-£200+ | Pin cleaning, connector repair, or harness work |
| Timing-related repair | Varies widely | Needed when the belt, chain, or tone wheel is part of the fault |
The reason I keep the timing-related row vague is simple: once the fault reaches that level, the price depends heavily on engine layout and how far the strip-down goes. A cheap sensor is not the expensive part of that story; the labour is. That is why a proper diagnosis matters before any parts are ordered.
What I would check before buying the sensor
If I were dealing with this fault on my own car, I would work through three questions before spending money on parts:
- Is the ECU seeing engine speed while cranking?
- Do power, ground, and signal reach the sensor correctly?
- Does the tone wheel or mechanical timing still look healthy?
If those checks are clean and the code returns, the sensor itself moves much higher on the suspect list. If they are not clean, replacing the sensor first is just a guess. That is the mistake that turns a manageable fault into an expensive one, and it is the one I would avoid every time.
