P0399 Cylinder 1 Pressure Sensor Circuit Intermittent/Erratic

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.

Likely causes

Symptoms

• Check engine light on
• Occasionally rough running
• Often no symptoms, the code comes and goes

Repair estimate

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

Related codes

References: saabwisonline.com, saabwisonline.com, law.cornell.edu.