Powertrain, generic (extended)

P2A3B O2 Sensor Negative Current Control Circuit/Open Bank 1 Sensor 3

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modeNo

P2A3B points to a break in the negative current control line of wide-band sensor 3 on bank 1, the common return of pump and Nernst cell. Bosch calls it virtual ground (IPN/VM, pin 2 on the LSU 4.9), NTK names it COM; the controller holds it at a bias of about 2.5 V, as Saab's 2010 9-5 shows at the plug, so current can flow both ways. No manufacturer documents P2A3B publicly. On the upstream analog P2251, Ford tells an open return from an open pump lead because lambda still shifts during reference-current pulses, and it flags a sensor voltage parked inside a band of 1.45–1.55 V, allowing 10 s before the fault is stored. P2A3C/P2A3D are low/high, P2A38 is the open pump lead. Expect the lamp, not a drivability change.

Likely causes

Broken virtual-ground wire or loose pin in the sensor plug
Corroded terminals in the connector near the rear exhaust
Internal break of the common electrode inside the probe

Symptoms

  • Check engine light
  • Sensor 3 signal stuck around its mid voltage in live data
  • Catalyst or aftertreatment monitor not completed

Repair estimate

DifficultyMediumDIY possible
Parts and labour€0–450
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

P2A3CP2A3DP2A38P22B2P2251P0142

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