Signal wire for the cylinder 1 pressure sensor shorted to a voltage supply or to another circuit; Faulty cylinder 1 pressure sensor giving a falsely high output; Corroded, damaged or loose connector terminals at the sensor or control unit; Open circuit or poor sensor ground, which can pull the signal high; Chafed or heat-damaged wiring harness near cylinder 1; Faulty sensor signal conditioning module, where fitted; Internal fault in the engine control unit.

P0398

Cylinder 1 Pressure Sensor Circuit High.

Generic
Powertrain
Generic code
Medium severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
3: AreaIgnition system or misfire
98: FaultThe specific fault in that area

What it means

P0398 means the engine control unit has seen a signal from the cylinder 1 pressure sensor circuit that is higher than the expected range. The sensor measures combustion or cylinder pressure, and the control unit may use it to fine-tune fuelling and timing. The code sets when the signal voltage stays above the upper limit, which usually points to a short to voltage, a wiring fault or a faulty sensor. The sensor type, location and strategy depend on the engine, so check manufacturer data for your model.

Symptoms

Engine warning light on
Rough running or uneven idle, especially on cylinder 1
Reduced power or the engine running in a limited performance mode
Poor fuel economy
Hesitation or harsher combustion noise under load
Possible hard starting on some engines

Possible causes

Signal wire for the cylinder 1 pressure sensor shorted to a voltage supply or to another circuitlikely
Faulty cylinder 1 pressure sensor giving a falsely high outputlikely
Corroded, damaged or loose connector terminals at the sensor or control unitlikely
Open circuit or poor sensor ground, which can pull the signal highpossible
Chafed or heat-damaged wiring harness near cylinder 1possible
Faulty sensor signal conditioning module, where fittedrare
Internal fault in the engine control unitrare

How to diagnose

Read all stored and pending codes and the freeze frame data, noting engine load, speed and temperature when the fault set.
Check for related codes covering sensor supply voltage, other cylinder pressure sensors or misfires, as they may point to a common cause.
Look over the wiring and connector at the cylinder 1 sensor for chafing, heat damage, oil contamination, moisture and bent or corroded terminals.
Look up the wiring diagram for your model to find the sensor supply, signal and ground circuits, and any signal conditioning module.
With the ignition on, measure the sensor supply voltage and the signal voltage against manufacturer specification.
Test the sensor ground for continuity and voltage drop.
Disconnect the sensor and check the signal wire for a short to voltage, then check the harness continuity back to the control unit.
View live data for cylinder 1 and compare it with the other cylinders, if the engine has more than one sensor, then clear the code and retest.

How to fix

Repair or replace any wiring that is shorted to voltage, damaged or open circuit.
Clean or replace damaged connectors and terminals, and make sure the sensor is fully seated and locked.
Replace the cylinder 1 pressure sensor if it is confirmed faulty, following the manufacturer procedure and torque.
Restore the sensor ground connection if it is poor or open.
Replace a faulty signal conditioning module or control unit only after the wiring and sensor are proven good, and carry out any adaptation the manufacturer requires.

Can I keep driving?

Medium severity

The car can usually be driven for a short time, but have it checked soon because combustion control may be degraded and the engine may enter a limited performance mode. Stop driving if it runs very roughly or loses power badly.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.