Damaged, chafed or corroded wiring to the cylinder 1 pressure sensor; Loose, corroded or water-contaminated connector at the sensor or control unit; Faulty cylinder 1 pressure sensor (internal fault or heat damage); Sensor contaminated with carbon or deposits, or poorly seated in the cylinder head; Electrical interference or a poor earth or reference voltage supply to the sensor; Underlying combustion problem in cylinder 1, such as a misfire, giving an erratic pressure signal; Fault inside the engine control unit or its sensor input circuit.

P0399

Cylinder 1 Pressure Sensor Circuit Intermittent/Erratic.

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
99: FaultThe specific fault in that area

What it means

P0399 means the engine control unit has seen the signal from the cylinder 1 pressure sensor circuit drop out, jump about or behave erratically. This sensor measures combustion pressure in the cylinder, and the ECU uses it to monitor and fine-tune combustion. The code sets when the signal is intermittent or implausible compared with what the ECU expects. Only some engines have in-cylinder pressure sensors, so how the code shows up depends on the model and engine.

Symptoms

Engine warning light on
Rough running or uneven idle
Hesitation or reduced power
Poor fuel economy
Difficult or extended starting
Engine running in a reduced-performance (limp) mode on some applications

Possible causes

Damaged, chafed or corroded wiring to the cylinder 1 pressure sensorlikely
Loose, corroded or water-contaminated connector at the sensor or control unitlikely
Faulty cylinder 1 pressure sensor (internal fault or heat damage)possible
Sensor contaminated with carbon or deposits, or poorly seated in the cylinder headpossible
Electrical interference or a poor earth or reference voltage supply to the sensorpossible
Underlying combustion problem in cylinder 1, such as a misfire, giving an erratic pressure signalpossible
Fault inside the engine control unit or its sensor input circuitrare

How to diagnose

Read all stored and pending codes and note the freeze frame data, including engine speed, load and temperature when the fault set.
Check for other related codes, such as misfire, injector or ignition faults on cylinder 1, as these may point to the real cause.
Inspect the sensor, its wiring and the connector for chafing, heat damage, corrosion, moisture and bent or pushed-back pins.
Wiggle-test the harness and connector with the engine running while watching the live cylinder 1 pressure signal for dropouts or spikes.
Check the sensor supply, reference voltage and earth against the manufacturer's data for your model.
Test the wiring for continuity, short to earth, short to supply and high resistance between the sensor and the control unit.
Check that the sensor is correctly fitted and free of heavy deposits, following the procedure for your engine.
If the wiring and supplies are good, compare the sensor signal with a known good reference or swap-test where the manufacturer allows, then consider the control unit.

How to fix

Repair or replace damaged wiring, and secure the harness away from heat and sharp edges.
Clean, repair or replace corroded or loose connectors and terminals.
Replace the cylinder 1 pressure sensor if it is faulty, and fit it using the correct torque and seals.
Correct any underlying cylinder 1 combustion fault, such as an ignition or injector problem, then recheck the signal.
Repair the supply or earth fault, and only where a known fault or update exists for your model, have the control unit software updated or the unit repaired or replaced.

Can I keep driving?

Medium severity

The car can usually be driven carefully for a short time, but get it checked soon. If the engine runs roughly, loses power or the warning light flashes, stop driving and have it inspected to avoid damage.

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