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P0318Rough Road Sensor A Signal Circuit

The signal from the rough-road sensor "A" circuit is missing, implausible, or out of range.

Definition

The rough-road sensor (an accelerometer, or wheel-speed data on some designs) tells the PCM when the vehicle is on broken pavement so the misfire monitor can be suspended instead of misreading suspension-induced crankshaft speed changes as misfires. P0318 sets when the sensor "A" signal is open, shorted, or outside its plausible range. The engine runs normally, but the MIL illuminates and misfire detection accuracy suffers — the vehicle may set false P0300-series codes on rough surfaces or, conversely, mask real misfires, and the stored code will fail a plug-in emissions inspection.

Common causes

  • Failed rough-road/acceleration sensor
  • Open or shorted sensor signal wiring, chafed harness, or corroded connector
  • Loss of the ABS wheel-speed message on vehicles that derive rough-road data from the ABS module
  • Poor sensor ground or missing 5 V reference
  • PCM input fault (rare)

Common misdiagnoses

  • Spark plugs and coils replaced chasing false misfire codes caused by the dead rough-road input
  • Wheel speed sensors replaced on vehicles with a dedicated accelerometer
  • PCM replaced before the sensor circuit is tested
  • Crankshaft position sensor replaced although its signal is normal

Troubleshooting steps

  1. 1.Check for companion codes

    ABS, U-series, or P0317/P0319 codes alongside this one indicate a shared network or supply problem rather than the sensor itself.

  2. 2.Locate and inspect the sensor and connector

    The accelerometer is typically body- or floor-mounted; look for water intrusion, green pins, and harness damage near the mounting point.

  3. 3.Verify reference, ground, and signal

    Expect a 5 V reference and a signal that idles near mid-range (about 2.5 V on many analog designs) and modulates when the body is bounced.

  4. 4.Wiggle-test the harness while watching live data

    A signal dropout during harness manipulation pinpoints an intermittent open or short in the wiring rather than the sensor.

  5. 5.Substitute the sensor if the circuit proves good

    With reference and ground verified at the connector and no response on the signal line, replace the rough-road sensor and confirm the monitor runs to completion.

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