Engine

P0398 — Cylinder 1 Pressure Sensor Circuit High

Severity: medium Drive with care, to a shop

What P0398 means

The engine control module monitors the cylinder 1 pressure sensor, which reports combustion conditions in that cylinder. P0398 sets when the signal voltage stays above the expected range. The cause can be the sensor itself, a wiring short to power or open circuit, a poor connector contact, or less often a fault in the ECM input.

Symptoms

  • Check engine light on
  • Rough or uneven idle
  • Cylinder 1 misfire indication
  • Reduced power and sluggish acceleration
  • Worse fuel economy

Possible causes

  1. Failed cylinder 1 pressure sensor
  2. Damaged wiring harness or corroded connector pins
  3. Signal wire shorted to power
  4. Intermittent wiring or connector fault from vibration or moisture
  5. ECM input circuit fault or software issue

How to diagnose

  1. Scan for codes and freeze frame data, and note any other cylinder or sensor codes
  2. Watch live data for the cylinder 1 sensor voltage with key on and with the engine running
  3. Inspect the sensor, connector and harness for chafing, oil, moisture, corrosion and bent pins
  4. Unplug the sensor and recheck the voltage: if it stays high, look for a short to power in the harness
  5. Test sensor supply, ground and resistance with a multimeter against the manufacturer specs
  6. Check the signal wire to the ECM for opens and shorts, wiggling the harness while testing
  7. Look for ECM software updates for your vehicle
  8. If the wiring and sensor are good, test the ECM

Common fixes

  • Repair the wiring or replace the damaged connector
  • Replace the cylinder 1 pressure sensor
  • Update or reflash the ECM software
  • Replace or repair the ECM if an input fault is confirmed

Questions and answers

What does P0398 mean?

The ECM is getting too high a voltage from the cylinder 1 pressure sensor circuit. The problem is in the sensor, its wiring or the ECM input.

Can I drive with code P0398?

Limited driving is usually okay if the engine runs smoothly. If it misfires, jerks or the check engine light flashes, avoid driving and have it checked soon.

How can I tell if it's the sensor or the wiring?

Unplug the sensor and watch the signal voltage. If it stays high, the fault is in the wiring or ECM; if it changes, the sensor is the more likely culprit.

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.