P0343
PowertrainCamshaft Position Sensor "A" Circuit High Input
Most of the time this comes down to the camshaft position sensor itself failing or a corroded connector that's gone high-resistance, often after oil works its way into the plug. The ECU watches the voltage coming back from the Bank 1 cam sensor, and when that signal sits higher than the window it expects, it logs P0343. The cam signal is what the engine uses to know where the valves are in relation to the crank, so when it goes wrong you get poor starting and rough running. On the high side specifically, it usually points at wiring or connector trouble rather than the sensor element wearing out.
ⓘ Information only. This page provides general educational information about fault code P0343. 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 P0343 mean?
P0343 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Camshaft Position Sensor "A" Circuit High Input.
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 a flickering or intermittent feel to the running
- • Long crank on a cold start, occasionally needing a second or third attempt before it catches
- • Rough, lumpy idle that smooths out once revs come up
- • Hesitation or a flat spot when you put your foot down, worst when the engine's loaded going uphill or pulling away
- • Random misfire feel and a drop in power on the motorway
- • On the bad ones the engine cuts out at idle or drops into limp mode
Possible causes
Causes commonly associated with P0343, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. The cam sensor itself has failed and is sending a high signal. Heat and oil contamination kill these over time, common on higher-mileage cars
- 2. Corroded or oil-fouled connector at the sensor plug. Oil tracks down the loom into the connector, pushes the resistance up and the voltage with it. This is a very common find
- 3. Chafed or broken signal wire shorting to a 12V supply, which drags the reading high. Look where the loom passes near the timing cover or any sharp edge
- 4. Damaged or stretched wiring harness from heat or vibration near the engine
- 5. Wrong supply voltage reaching the sensor, 12V where the sensor wants a 5V reference
- 6. Internal ECU input fault, rare and only worth considering once everything else checks out clean
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. Hook up a scanner and watch the cam sensor live data while you crank and idle. A high or pegged voltage that won't track the engine speed tells you the signal's stuck up where it shouldn't be
- 2. Unplug the sensor connector and have a proper look inside. Oil in the pins, green corrosion or a spread terminal will cause exactly this code. Clean it with contact cleaner and recheck before you condemn anything
- 3. Back-probe the signal and reference wires with a multimeter, key on. Confirm the sensor's getting its correct reference voltage, usually 5V, and that the signal isn't sitting pinned at supply voltage
- 4. Wiggle-test the loom from the sensor back toward the ECU connector while watching live data. A signal that jumps as you move the wiring points straight at a chafe or break
- 5. Check the timing cover area for an oil leak feeding the connector. No point fitting a new sensor into an oily plug, it'll be back inside a week
- 6. Clear the code, run it through a proper drive cycle with a cold start, and rescan to see if it returns or whether it was a one-off glitch
Common questions about P0343
How do I know if it's the sensor or the wiring on my car? +
Start at the connector. Unplug it and look for oil or corrosion in the pins, because a fouled or high-resistance plug throws this code without the sensor being faulty at all. Clean it, refit it and see if it clears. If the connector's dry and tidy, back-probe the signal wire with a multimeter, key on engine off. A reading pinned up near 12V on a 5V reference sensor means the signal's shorted to power somewhere in the loom. If the supply and signal voltages look right but the live data still reads high while cranking, the sensor itself is the likely fault. Wiggle the loom while watching the data to catch an intermittent break.
Can I just clean it rather than buy a new sensor? +
Worth a try, and it works on a fair number of these. If oil or moisture has got into the connector, a good clean with contact cleaner and reseating the pins can clear the high reading completely. Fix the oil leak that put it there first, otherwise you're back to square one. If the sensor element has actually failed or the wire is shorted, cleaning does nothing and you'll need to replace the part or repair the loom. Sensors are usually £20 to £80 and a five-minute job on accessible engines.
If I clear the code does it stay gone? +
Depends what caused it. A genuine wiring short or a dying sensor will bring the light straight back, often within a few miles or on the next cold start. If it was a damp or dirty connector and you've cleaned it properly, clearing the code and driving it through a full cycle should see it stay off. Clearing without fixing the root cause just buys you a day or two. If it keeps returning after a clear, stop resetting it and diagnose it properly, the code is telling you something real.
What happens if I leave it? +
It can get worse fast. A failing cam signal makes the engine hard to start and rough to run, and if the sensor drops out entirely the engine may stall or refuse to start, sometimes leaving you stranded at a junction or on the motorway. Some engines fall back on the crank sensor and limp along, others won't run at all without a cam signal. There's also the misfire risk to think about, and sustained misfiring is no friend to your catalytic converter. Get it diagnosed before it goes from an annoying light to a no-start.
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 →