Powertrain, generic (extended)

P25F4 Turbocharger/Supercharger Boost Sensor C Circuit Low

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modePossible

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.

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

Symptoms

  • Check engine lamp on
  • Noticeable loss of power
  • Scan tool shows a pressure far below ambient for sensor C

Repair estimate

DifficultyMediumDIY possible
Parts and labour€40–400
Time0.5–2 h

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

Parts involved

boost pressure sensor cwiring harness

Related codes

P25F2P25F3P25F5P0237P0241

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