P0335 Crankshaft Position Sensor: Why the Engine Cuts Out

P0335 is a circuit code. It says the ECU is not receiving a usable signal from crankshaft position sensor A, and nothing about why. The sensor is one candidate; so are its connector, its wiring, the toothed reluctor wheel it reads, and metallic debris on the sensor tip. On most cars the crank signal is the reference the whole engine runs from, which is why this fault announces itself as a no-start or a cut-out rather than a quiet warning light.

What the code actually means

The crankshaft position sensor sits close to a toothed wheel, usually on the crank itself, on the flywheel, or behind the front pulley. As the teeth pass, the sensor produces a stream of pulses, and a deliberate gap in the teeth acts as the reference mark telling the ECU exactly where the crankshaft is.

From that pulse train the ECU derives engine speed, crank angle, ignition timing and injection timing, and it uses variations in pulse spacing to detect misfires. It is the most fundamental input the engine has. Interrupt it and the ECU has no idea when to fire anything, so on most cars it stops trying.

Two sensor types are common. An inductive sensor generates its own alternating voltage as the teeth pass and needs no supply. A Hall effect sensor is fed a supply and a ground and switches a signal line, which adds two more things that can be at fault.

The word circuit matters. The ECU judges the signal arriving at its own pin, at the end of a loom that may be a metre long, through a connector in the hottest, dirtiest part of the engine bay. It cannot tell a dead sensor from a good sensor whose wire has chafed against a bracket. That distinction is yours, and it is the difference between a few pounds and a part fitted for nothing.

One cause worth knowing: the reluctor ring is steel and the sensor tip is often magnetic. Swarf from a wearing clutch, gearbox or thrust bearing collects on the tip until the pulses smear together. The sensor tests fine on the bench, wipes clean in seconds, and the code stays away for months.

Symptoms you’ll actually notice

This is not a code that hides behind a warning lamp. The typical experience is a car that is fine one minute and dead the next.

• Cranking with no start at all, the engine turning over normally but never catching
• Sudden cut-out while driving with no warning, then a restart minutes later as if nothing happened
• Long cranking before it fires, particularly on a warm restart
• Intermittent stalling at junctions or once the engine is fully up to temperature
• On some cars, the rev counter dropping to zero or flickering as it cuts out, which is a strong clue
• A misfire code alongside it, because misfire detection depends on the very signal that has become unreliable

Likely causes, cheapest first

Order matters more here than for most codes, because the sensor is cheap enough that people fit one immediately and then have no plan when the fault returns. Half an hour spent looking at the tip, the plug and the loom either finds the problem or rules out three of the six rows above.

The heat-related intermittent

The most frustrating version of P0335 is the sensor that only misbehaves hot. The car starts perfectly every morning, drives normally, then cuts out after forty minutes on the motorway or refuses to restart after a supermarket stop. An hour later it fires up as though nothing happened, and any garage that tests it cold finds nothing wrong.

The mechanism is simple. As the sensor’s winding or electronics warm through, a marginal joint opens up or the signal amplitude drops below what the ECU will accept. Cool it and the fault closes again. The same behaviour comes from a connector whose pin tension has gone, or a wire whose broken strands separate as the loom expands.

That is why freeze frame is so valuable here. If the snapshot shows coolant temperature at full operating value every time the fault sets, you have confirmed a heat-related problem and you know to test the car hot rather than in a cold workshop bay.

Narrowing it down with live data

The single most useful parameter is engine RPM as reported by the ECU, watched while the engine is cranking. On a healthy car it rises to a steady cranking figure immediately. If RPM reads zero or jumps around wildly while the starter is clearly turning the engine, the ECU is not getting a clean crank signal, and you have separated a crank sensor problem from a fuel or spark problem in about ten seconds.

Watch RPM at idle as well, with the engine hot. A signal that is degrading rather than dead often shows as a rev reading that flickers or briefly drops while the engine still sounds smooth. That is the fault developing in front of you.

Check whether a camshaft position code is stored alongside, and note coolant temperature and calculated load in the freeze frame to complete the picture of when the fault appears. Live data and freeze frame need the Enhanced edition or above; the Standard licence reads and clears codes but does not stream sensor values, which is the one capability this fault really wants. The edition comparison lays out the difference.

Garage bill versus doing it yourself

Diagnostic work in the UK is usually charged at around one hour of labour, and rates differ sharply between a small independent and a franchised dealer, and between regions. Access is the wild card here: on some engines the sensor is one bolt behind the alternator, and on others it sits between the engine and the bulkhead, or at the bellhousing where the car needs lifting. The same part number can mean twenty minutes or three hours.

When this is a DIY job and when it isn’t

Reasonable at home

If you can see and reach the sensor from above with the bonnet up, this is a fair job. One bolt, one connector, and often an O-ring that should be renewed rather than reused. Note the exact orientation before removal, keep the mounting face clean, and do not lever the sensor out against the reluctor ring. Some engines specify a set air gap between tip and teeth, so check whether yours does.

Leave it alone

Sensors at the bellhousing, behind the exhaust manifold or under the inlet need a lift and the removal of other components, and a snapped sensor body left in the block turns a cheap job into a very expensive one. Anything involving the reluctor ring is gearbox-out territory. If the diagnosis points at wiring rather than the sensor, a garage with a scope will find the break far faster than you will by eye.

Clearing the code the right way

Repair first, then clear, then test deliberately. Because this fault is so often intermittent, a code that stays away for ten minutes proves nothing. Clear it, drive the car until it is thoroughly hot, sit it at idle for a while, switch off, leave it fifteen minutes and restart. Recreating the freeze frame conditions is the only meaningful test.

If the code returns, that is information rather than failure. A P0335 that comes straight back after a new sensor is telling you the sensor was never the problem. Pending codes help too: the ECU logs the fault as pending on first sight and confirms it on the second, so a pending code after a repair means the fault is still there and only just.

Clearing also resets the emissions readiness monitors, so the car needs a spread of cold starts, town driving and steady cruising before those complete. Never clear this one simply to get the light out; a car that can cut its engine at a roundabout is a safety problem, not a cosmetic one.

What it means for your MOT

An illuminated engine management light is an MOT failure in the UK on its own account, whatever the code behind it, on petrol cars first used from July 2003 and diesels from July 2008. (The same applies at a US emissions or smog check, where a lit lamp or unfinished readiness monitors both fail.) Clearing the code the day before will not help, because monitors that have not completed are themselves a fail. Fix the circuit, let the car finish its self-checks over a couple of days of ordinary driving, and present it with the light out honestly.