Body, generic

B0255 HVAC Air Temperature External Sensor Circuit

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modeNo

B0255 shows up in no publicly available J2012 source. Saab's 9-3 uses it for a door motor: B0255 06 reports the right vent door motor circuit open or grounded, B0256 01 the B+ short on that motor. By its title, though, the code means the outside air sensor; that component also has the codes B0158 and B0159. HELLA's ambient sensor is an NTC resistor read through a voltage divider, rated −40 to +65 °C with a time constant under 35 s, and the module turns its resistance into a temperature using the sensor curve. Mounted behind the grille or under a mirror, it suffers from stone chips, salt and pinched wiring. Climate control uses the value to choose mode and fan speed, so a fault means poor regulation and a blank or wrong outside temperature.

Likely causes

Outside sensor broken by stone impact or corroded by road salt
Sensor wiring behind the bumper pinched or torn, often after front-end repairs
Moisture in the sensor plug changing the measured resistance

Symptoms

  • Outside temperature display shows dashes or an implausible value
  • Automatic climate control regulates poorly, especially in cold or hot weather

Repair estimate

DifficultyEasyDIY possible
Parts and labour€25–150
Time0.2–1 h

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

Parts involved

ambient temperature sensorhvac control module

Related codes

B0158B0159B0250

References: saabwisonline.com, hella.com.