Powertrain, generic · Ignition system or misfire

P0318 Rough Road Sensor A Signal Circuit

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modeNo

Misfire detection works from crankshaft speed fluctuations, which rough roads also cause. CARB therefore allows misfire detection to pause on rough roads; many engines take a rough-road signal from the wheel speeds of the ABS module, a Ford patent describes such a detector. P0318 means rough-road signal A is missing or invalid. On GM engines (documented in Saab service information for the 1.6 turbo) the brake control module sends wheel speed over the data bus; the code sets between 1,520 and 4,000 rpm when that module reports a wheel speed sensor fault or no signal arrives for more than 5 s. GM files it without the warning light. Misfire detection then runs without its rough-road filter. ABS or stability warnings and further codes often appear at the same time.

Likely causes

Wheel speed sensor or wiring faulty – the brake module passes on the fault
Communication between engine and brake module lost (CAN, connector, supply)
Brake control module faulty
Separate rough-road sensor faulty, where fitted

Symptoms

  • Service message (GM without warning light)
  • Often ABS or stability warning at the same time
  • Driving unaffected

Repair estimate

DifficultyMediumDIY possible
Parts and labour€50–400
Time0.5–2 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

wheel speed sensorsabs modulecan bus

Related codes

P0317P0319P0300P0500U0121

References: saabwisonline.com, law.cornell.edu, gsi.ext.gm.com.