Engine

P03AC — Cylinder 3 Pressure Sensor Circuit High

Severity: medium Drive with care, to a shop

What P03AC means

The engine control module reads the cylinder 3 pressure sensor signal and compares it to an expected range. P03AC sets when the voltage is higher than it should be. Typical causes are a failed sensor, a short to voltage in the wiring, a poor connector contact, or, less often, an ECM fault.

Symptoms

  • Check Engine light on
  • Rough idle
  • Hesitation and reduced power
  • Possible cylinder 3 misfires
  • Worse fuel economy

Possible causes

  1. Failed cylinder 3 pressure sensor
  2. Signal wire shorted to voltage
  3. Open circuit or poor ground in the sensor circuit
  4. Corroded or damaged connector pins
  5. ECM malfunction
  6. Truly abnormal cylinder pressure (rare)

How to diagnose

  1. Scan all codes and review freeze frame data, noting any other cylinder 3 codes
  2. Inspect the sensor harness and connector for chafing, moisture, corrosion or heat damage
  3. Check sensor supply and reference voltage at the connector with a multimeter, using the wiring diagram
  4. Test ground and signal wire continuity and look for a short to voltage
  5. Compare live sensor data with the other cylinders on a scan tool
  6. Wiggle the harness with the engine running and watch for signal changes
  7. If mechanical trouble is suspected, run a compression test on cylinder 3
  8. If wiring and sensor are good, evaluate the ECM and check for software updates

Common fixes

  • Repair damaged wiring and clean or replace the connector
  • Replace the cylinder 3 pressure sensor
  • Reflash or replace the ECM if it is confirmed faulty
  • Fix any mechanical cause of abnormal cylinder pressure

Questions and answers

What does P03AC mean?

It means the cylinder 3 pressure sensor signal is above the expected range. The sensor, its wiring or the connector are the most common culprits.

Can I drive with P03AC?

Briefly and carefully, if the engine runs without misfires. Do not put off diagnosis, to avoid engine and catalytic converter damage.

How do I clear P03AC?

Fix the cause first, then clear the code with a scan tool. Without a repair, the code will return.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.