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 3. P03AC means the sensor signal stays above its upper limit. GM's European 2.0 CDTI diesel (Saab 9-5, 2010) sets it when the signal is above the threshold for more than 2 s with the engine running, checked once per drive cycle, with the check engine light after two trips; the displayed maximum combustion pressure then reads 10,000 kPa instead of the normal 0–9,000 kPa. Saab names a 5 V reference shorted to positive, an open signal wire or one shorted to positive, and an open ground wire as causes – an unplugged connector is the most common. Without the signal, combustion control for this cylinder is switched off, which is usually barely noticeable.
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 3 is numbered according to the manufacturer: check your engine's manual before working on it.
Symptoms
- Check engine light on (often after two trips)
- Usually barely noticeable – combustion control for the cylinder is switched off
- Possibly rougher running, more noise or slightly higher consumption
Most likely causes
- Connector off or loose, signal wire open
- Sensor ground wire open
- Sensor in the glow plug faulty
- Signal or reference wire shorted to positive
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 P03AC means
- P: Powertrain: engine and transmission
- 0: Generic code: same on every make
- 3: Ignition and misfires
- AC: Specific fault in that system
Frequently asked questions
Can I keep driving with code P03AC?
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 P03AC?
The most common causes are: connector off or loose, signal wire open; sensor ground wire open; sensor in the glow plug faulty.
Related codes
- P03A9Cylinder 3 Pressure Sensor Circuit
- P03AACylinder 3 Pressure Sensor Circuit Range/Performance
- P03ABCylinder 3 Pressure Sensor Circuit Low
- P03ADCylinder 3 Pressure Sensor Circuit Intermittent/Erratic
- P0398Cylinder 1 Pressure Sensor Circuit High
- P03A2Cylinder 2 Pressure Sensor Circuit High
Sources
- saabwisonline.com: dtc p0397 p0398 p03a1 p03a2 p03ab p03ac p03b5 or p
- law.cornell.edu: 13 CCR 1968.2
- borgwarner.com: pressure sensor glow plugs
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.