P0340 means the engine computer has lost or can’t reliably read the camshaft position sensor’s signal — the sensor that identifies which cylinder is on its power stroke, letting the ECU fire spark plugs and injectors in the correct sequence rather than just the correct timing generally. Many engines can technically limp along with this signal missing, defaulting to a less precise firing pattern, which is why P0340 more often causes rough running than a complete no-start — a useful distinction from its close cousin, P0335.
What Does P0340 Mean, and How Is It Different From P0335?
The camshaft position sensor identifies which specific cylinder is on its power stroke, working alongside the crankshaft position sensor to give the ECU a complete picture of engine rotation. The crank sensor establishes overall rotational position and speed; the cam sensor adds which cylinder is which, needed for sequential fuel injection and coil-on-plug ignition to fire the correct components at the correct moment.
Many engine control systems can fall back to a less precise ‘batch fire’ or wasted-spark strategy if camshaft position data is lost, since crankshaft data alone still allows the engine to run — just less efficiently and often roughly. This is why P0340 more commonly produces a rough-running symptom than the complete no-start P0335 often causes, though a total loss of both signals together does typically prevent starting.
Common Symptoms
- Rough idle or hesitation, from imprecise fuel and spark timing.
- Misfires — see our P0300 guide if a misfire code accompanies this one.
- Difficulty starting, though often less severe than a complete no-start.
- Reduced fuel economy from a less efficient firing strategy.
- Intermittent stalling in more severe cases.
Common Causes
- A failed camshaft position sensor — the most common cause.
- Damaged or corroded wiring to the sensor.
- Oil contamination at the sensor tip from a nearby valve cover gasket or seal leak.
- Excessive air gap from a previous improper installation, or timing chain/belt stretch affecting the sensor’s relationship to the camshaft.
- A failed sensor connector, exposed to heat and vibration over years of service.
How to Diagnose It
- Check for oil leaks near the sensor, particularly from the valve cover gasket, since contamination is a genuinely common contributing cause.
- Test sensor resistance and signal against manufacturer specification.
- Inspect wiring and the connector for damage or corrosion.
- Check for related codes, particularly P0335, since both together point at a more fundamental timing issue rather than one sensor in isolation.
Never work under a vehicle supported only by a jack. Use axle stands on firm, level ground, chock the wheels that stay down, and leave the car in gear or Park with the handbrake on. A jack lifts; stands hold.
Possible Fixes
| Cause | Fix | Typical cost |
|---|---|---|
| Failed camshaft position sensor | Replace the sensor | $100 – $250 |
| Damaged wiring | Repair the affected section | $80 – $250 |
| Oil-contaminated sensor | Clean sensor, fix the leak source | $100 – $350 |
| Timing chain/belt stretch | Replace as needed | $400 – $1,200 |
Can I Drive With P0340?
For short trips, with caution — rough running and reduced fuel economy are the more likely symptoms rather than a sudden stall, but the fault can worsen over time and is worth diagnosing reasonably promptly. If misfires accompany it, treat that combination with more urgency per our check engine light flashing guide, since active misfires risk catalytic converter damage.
When to See a Mechanic
Book it in reasonably promptly, particularly if rough running or reduced fuel economy is noticeable — more urgently if stalling has occurred even once. More in our trouble codes section.
Frequently Asked Questions
What does P0340 mean?
The engine computer has lost or can’t reliably read the camshaft position sensor’s signal — the sensor that identifies which cylinder is on its power stroke for correct fuel and spark sequencing.
What’s the difference between P0340 and P0335?
P0335 is the crankshaft position sensor, establishing overall engine rotation; P0340 is the camshaft sensor, identifying which specific cylinder is which. Losing the cam signal alone often causes rough running rather than a complete no-start, since some systems can fall back to a less precise firing strategy.
Can P0340 cause my car not to start?
It can, though it more commonly causes rough running or difficulty starting rather than a complete no-start, since many systems can partially compensate using crankshaft data alone.
Is it safe to drive with a P0340 code?
For short trips, with caution — it’s less likely to cause sudden stalling than a crankshaft sensor fault, but it’s still worth diagnosing reasonably promptly rather than living with it.
How much does it cost to fix P0340?
Often $100–$250 for a sensor replacement, more if oil contamination or a timing component turns out to be the actual cause.
