P0399
Cylinder 1 Pressure Sensor Circuit Intermittent/Erratic
Combustion pressure sensing
Generic OBD-II
High
What it means
P0399 means the ECM sees an intermittent or erratic signal from the cylinder 1 combustion pressure sensor — a sensor that monitors in-cylinder pressure to help fine-tune combustion. The engine usually keeps running, but the fault can limit combustion-based controls and cause rough running, so the sensor or its wiring should be traced and repaired.
High
Can I keep driving? The engine generally runs, but combustion-based controls may be limited, so you can see reduced performance or rough running until it's fixed.
Common symptoms
What you'll notice
- Check-engine light on
- Rough or uneven running
- Reduced power or hesitation
- May behave normally between intermittent dropouts
Common causes
Most likely first
- Faulty cylinder 1 combustion pressure sensorLikely
- Intermittent open, short, or chafed wiring in the sensor circuitCommon
- Poor or corroded connection at the sensorCommon
- Loose sensor mounting affecting the signalLess often
- Faulty ECM (least likely)Less often
How to diagnose & fix
Work through it in order
- Read freeze-frame data and confirm the fault is the cylinder 1 pressure sensor circuit.
- Wiggle-test and inspect the sensor connector and harness for intermittent contact.
- Check the signal, reference, and ground at the sensor against specification.
- Confirm the sensor is properly seated and torqued.
- If the circuit is good, replace the cylinder 1 pressure sensor. Clear and re-test.
FAQ
Quick answers
Is it safe to drive with P0399?+
Short-term, usually — this is a monitoring sensor rather than a fuel or spark control, so the engine keeps running, though performance may be limited. Repair it to restore normal combustion control.
Can I just clear P0399?+
It will return when the intermittent fault reappears. Clearing only resets the light.
Related codes
Often seen with P0399
Read & clear P0399 yourself
Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.
✓ Verified name — SAE J2012 / ISO 15031-6Guidance reviewed by OBD DashX