Engine

P0399 — Cylinder 1 Pressure Sensor Signal Intermittent or Erratic

Severity: medium Drive with care, to a shop

What P0399 means

The engine control module monitors the cylinder 1 pressure sensor signal and has found that it drops out or jumps around in a way that does not match engine operation. These sensors are used on a limited number of engines, and the exact code definition can vary by manufacturer, so check the service information for your vehicle. The cause is most often electrical: wiring, a connector or the sensor itself.

Symptoms

  • Check engine light on
  • Rough idle and vibration
  • Loss of power or sluggish acceleration
  • Cylinder 1 misfire or knock
  • Intermittent stalling

Possible causes

  1. Failed cylinder 1 pressure sensor
  2. Damaged or corroded wiring and connector at the sensor
  3. Open circuit, poor contact, or short to ground or power in the signal wire
  4. Fault in the ECM input circuit
  5. Mechanical problem in the cylinder affecting readings (uncommon)

How to diagnose

  1. Scan all codes and freeze-frame data, and look for misfire or sensor supply codes.
  2. Inspect the cylinder 1 sensor harness and connector for chafing, moisture, corrosion or heat damage.
  3. Wiggle the harness and connector with the engine running while watching the sensor signal on a scan tool.
  4. Check reference voltage, power and ground at the sensor with a multimeter, following the factory wiring diagram.
  5. Test the signal wire back to the ECM for opens and shorts.
  6. Compare the sensor reading to specification, or scope the signal if possible.
  7. If wiring and sensor are good, check cylinder compression and then suspect the ECM.

Common fixes

  • Repair the wiring, replace a damaged connector or clean the terminals
  • Replace the cylinder 1 pressure sensor
  • Reprogram or replace the ECM if an internal fault is confirmed
  • Repair a confirmed mechanical fault in the cylinder

Questions and answers

What does P0399 mean?

The engine computer is getting an intermittent or unstable signal from the cylinder 1 pressure sensor. Wiring, a connector or the sensor itself is the usual culprit.

Can I drive with P0399?

If the engine runs smoothly you can drive to a shop. If you notice shaking, power loss or stalling, avoid driving to protect the catalytic converter and stay safe.

Will clearing the code fix it?

No. A bad connection or failing sensor will set the code again. Find and fix the fault first, then clear the code.

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.