Powertrain, generic (extended)

P2A3C O2 Sensor Negative Current Control Circuit Low Bank 1 Sensor 3

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

P2A3C is stored when the virtual-ground return belonging to the third probe of bank 1 falls below its intended bias. Because this line normally floats around 2.5 V rather than at chassis potential, a chafed wire touching the body or an exhaust bracket drags it down and leaves the pump cell little room to reverse its current, so rich mixtures can hardly be measured. No manufacturer publishes P2A3C; this text relies on CARB's demand to detect out-of-range sensor circuits, on the Bosch LSU 4.9 pin layout and on the bias value Saab gives for its 2010 9-5 at the plug. P2A3B is the open return, P2A3D the high case, P2A39 the matching low fault on the pump lead. The lamp is lit and the downstream sensor checks will not complete.

Likely causes

Virtual-ground wire chafed through to chassis ground
Internal short from the common electrode to the sensor housing
Bias stage in the engine control module faulty

Symptoms

  • Check engine light after repeated detection
  • Downstream emission monitors stay incomplete

Repair estimate

DifficultyMediumDIY possible
Parts and labour€50–450
Time0.5–2.5 h

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

Parts involved

oxygen sensor b1s3wiring harnessecm

Related codes

P2A3BP2A3DP2A39P22B3P0143

References: bosch-motorsport.com, saabwisonline.com, law.cornell.edu.