Boost pressure sensors of this kind output a voltage that rises with pressure, so P25F4 – a signal below the lowest valid value – points to the wiring or the sensor rather than to the charge air. Usual reasons are a signal wire shorted to ground, a missing 5 V supply, or an open signal line on an input pulled down by the control unit. No public manufacturer data exist for sensor C; the GM-developed Saab engines give an idea for sensor A, setting the low code under 0.09 V for more than 5 s on the 2.0 litre and under 0.19 V for 4 s in one go or 50 s added up on the 1.6 litre. The control unit then typically disables boost control. The high-voltage counterpart is P25F5; a shared 5 V supply fault can bring up other sensor codes at the same time.
Medium severity. Can I keep driving?Drive without demanding power and go to a repair shop soon. If you see a lot of blue smoke or hear a metallic noise, shut off the engine.
Symptoms
- Check engine lamp on
- Noticeable loss of power
- Scan tool shows a pressure far below ambient for sensor C
Most likely causes
- Signal wire shorted to ground
- 5 V reference open or pulled down by another faulty sensor on the same supply
- Boost sensor C failed internally
What to check, in order
Start with the simplest. You can tick off each step.
Good to know
- Repair difficulty: medium
- It can trigger limp mode.
- It affects emissions.
What each character of P25F4 means
- P: Powertrain: engine and transmission
- 2: Generic code: same on every make
- 5: Speed and idle
- F4: Specific fault in that system
Frequently asked questions
Can I keep driving with code P25F4?
Drive without demanding power and go to a repair shop soon. If you see a lot of blue smoke or hear a metallic noise, shut off the engine.
What causes code P25F4?
The most common causes are: signal wire shorted to ground; 5 V reference open or pulled down by another faulty sensor on the same supply; boost sensor C failed internally.
Related codes
- P25F2Turbocharger/Supercharger Boost Sensor C Circuit
- P25F3Turbocharger/Supercharger Boost Sensor C Circuit Range/Performance
- P25F5Turbocharger/Supercharger Boost Sensor C Circuit High
- P0237Turbocharger/Supercharger Boost Sensor A Circuit Low
- P0241Turbocharger/Supercharger Boost Sensor B Circuit Low
Sources
- law.cornell.edu: 13 CCR 1968.2
- saabwisonline.com: dtc p0236 p0237 or p0238
- saabwisonline.com: dtc p0237 or p0238 llu
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.