Some diesels have a combustion pressure sensor in the glow plug; the engine module uses it to control injection for each cylinder. This code concerns cylinder 1. P0399 means the sensor signal is erratic or drops out for moments. GM's European 2.0 CDTI diesel (Saab 9-5, 2010) calls it 'circuit erratic' and sets it when the signal is outside its range for more than 0.2 s with the engine running; its fault table groups the erratic and pressure variation codes together as signal performance faults. The test plan checks a reference of 4.8–5.2 V and a ground below 5 Ω before the glow plug is replaced. Typical causes are a loose contact at the glow plug connector, a chafed engine harness or sensor electronics that become unstable with temperature. The code often comes and goes; combustion control for the cylinder is suspended while it is active.
Medium severity. Can I keep driving?If the car is running normally, drive in moderation to a repair shop. If it loses power or another warning appears, do not continue.
Cylinder 1 is numbered according to the manufacturer: check your engine's manual before working on it.
Symptoms
- Check engine light on
- Occasionally rough running
- Often no symptoms, the code comes and goes
Most likely causes
- Loose contact at the glow plug connector
- Chafing or broken wire in the engine harness
- Sensor electronics of the glow plug unstable with temperature
- Interference from ground problems
What to check, in order
Start with the simplest. You can tick off each step.
Good to know
- Repair difficulty: medium
- It affects emissions.
What each character of P0399 means
- P: Powertrain: engine and transmission
- 0: Generic code: same on every make
- 3: Ignition and misfires
- 99: Specific fault in that system
Frequently asked questions
Can I keep driving with code P0399?
If the car is running normally, drive in moderation to a repair shop. If it loses power or another warning appears, do not continue.
What causes code P0399?
The most common causes are: loose contact at the glow plug connector; chafing or broken wire in the engine harness; sensor electronics of the glow plug unstable with temperature.
Related codes
- P0395Cylinder 1 Pressure Sensor Circuit
- P0396Cylinder 1 Pressure Sensor Circuit Range/Performance
- P0397Cylinder 1 Pressure Sensor Circuit Low
- P0398Cylinder 1 Pressure Sensor Circuit High
- P03A3Cylinder 2 Pressure Sensor Circuit Intermittent/Erratic
- P03ADCylinder 3 Pressure Sensor Circuit Intermittent/Erratic
Sources
- saabwisonline.com: dtc p0399 p039c p039d p03a3 p03a6 p03a7 p03ad p03b
- saabwisonline.com: dtc p0397 p0398 p03a1 p03a2 p03ab p03ac p03b5 or p
- law.cornell.edu: 13 CCR 1968.2
Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.