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P0343 — Camshaft Position (CMP) Sensor — Circuit High Input

Moderate

Quick answer

P0343 means the cam sensor’s the signal is stuck high — typically a short to voltage, a broken ground or reference wire, or a failed sensor. Many engines still run on the crank signal alone — with longer cranking, reduced power, or batch-fired injection — so the symptom is often “runs, but not right.” On others it stalls or won’t start. Check the connector and wiring before buying a sensor — for this variant of the code, wiring is the most common answer.

What it means

The camshaft position (cmp) sensor tells the engine computer which cylinder is on its compression stroke, so the computer can sequence injectors and (on many engines) coil firing. P0343 sets when the signal is stuck high — typically a short to voltage, a broken ground or reference wire, or a failed sensor for long enough that the computer stops trusting it.

Circuit codes describe electrical behavior, which puts the wiring, the connector, and the sensor on equal footing as suspects. The cheapest of the three fails most often.

While the signal is untrusted, the computer substitutes a safe default value. The engine runs, but on assumptions instead of measurements — that’s the drivability change you feel.

Common causes

Ordered from most to least likely.

  1. 1.

    Wiring damage (chafe, break, melted insulation)

    A short to voltage or broken ground pins the signal high.

  2. 2.

    Corroded, loose, or backed-out connector pins

    Unplug and inspect both halves under good light.

  3. 3.

    Failed cam sensor

    Confirm with measurements before replacing.

  4. 4.

    Stretched timing chain / jumped timing

    Especially on engines known for chain wear — the sensor is reporting a real mechanical condition.

  5. 5.

    Lost 5V reference or sensor ground (where applicable)

    If several sensors fault together, suspect a shared reference circuit rather than coincidence.

How to diagnose it, step by step

Cheapest and most likely checks first.

  1. 1 Read the freeze frame

    Note when P0343 sets — cold start, warm idle, under load, over bumps. The conditions narrow the cause dramatically, especially for intermittent faults.

  2. 2 Inspect connector and harness

    Unplug the sensor; check for corrosion, bent or spread pins, and chafed insulation along the harness run. Re-seat firmly. This free step resolves a remarkable share of circuit codes.

  3. 3 Watch it in live data

    Scanners that show cam/crank correlation make this easy: the two signals must agree. A correlation error with a healthy circuit often means the timing chain has stretched — a mechanical story, not electrical.

  4. 4 Rule out the timing side

    Range/performance versions of this code frequently mean cam-to-crank timing is off: stretched chain, jumped tooth, or a failed variable-timing actuator. Check for VVT codes and listen for chain noise before condemning the sensor.

  5. 5 Wiggle-test if intermittent

    Engine running, data live: gently flex the harness and tap the sensor while watching the reading. A glitch you can provoke is a fault you can find.

  6. 6 Replace with a quality part

    If measurements condemn the sensor, buy OEM or a reputable brand — bargain sensors re-set these codes often enough to cost more in time than they save in money.

Parts & tools you may need

  • OBD-II scanner (code reader with freeze frame / live data)
  • Digital multimeter
  • Electrical contact cleaner
  • Replacement cam sensor (exact part for your engine)

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Related codes

Frequently asked questions

Can I drive with P0343?
Usually, in its degraded mode — but diagnose soon: if the real cause is timing drift, it only goes one direction.
Is it the sensor or the wiring?
For this variant, lean wiring: stuck-low, stuck-high, and intermittent signatures are circuit behaviors. Inspect and measure before buying the sensor.
Why did the code return after a new sensor?
Because the circuit, not the sensor, was the fault — or the replacement was low quality. Re-do the wiring inspection the first repair skipped.
What does the computer do meanwhile?
It substitutes a default value and keeps the engine running on assumptions. Functional, but you pay in drivability and fuel until the real measurement comes back.