Powertrain, generic (extended)

P2AC5 Intake Air O2 Sensor Reference Voltage Circuit Low Bank 2

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modeNo

P2AC5 reports that the bank 2 intake oxygen sensor sees a reference voltage below the permitted limit, typically a short to ground in the reference lead or a breakdown of the sensing cell. No manufacturer publishes this code, and engines with a separate intake oxygen sensor for bank 2 are rare. If a vehicle does set it, compare with P2AC1: a fault on only one bank points to that sensor or its own harness branch, while both banks together hint at a shared ground, the sensor supply or the ECM. While the reference is pulled low, the bank 2 EGR feedback is lost and the ECM is expected to reduce recirculation, which raises NOx output but rarely affects driveability. CARB and Ford's sensor patents frame the check. P2AC6 is the high side, P2AC3 the open circuit.

Likely causes

Reference wire of the bank 2 sensor shorted to ground
Sensing cell broken down internally
Shared sensor supply or ECM fault (if P2AC1 is present as well)

Symptoms

  • Check engine light on
  • Higher NOx, possibly a failed emissions test

Repair estimate

DifficultyMediumDIY possible
Parts and labour€0–350
Time0.3–2 h

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

Parts involved

intake air o2 sensor b2wiring harnessecm

Related codes

P2AC3P2AC4P2AC6P2AC1

References: patents.google.com, patents.google.com, law.cornell.edu.