P2251: Front oxygen reference-ground circuit fails its open-circuit check

P2251 is labeled negative-current-control circuit/open in the generic dictionary. The reviewed GM table more specifically identifies the bank 1 sensor 1 reference-ground signal circuit. This terminology difference matters when selecting the correct wide-range sensor circuit information.

What the interface detects

In the ATIC142 implementation, an open reference-ground indication can result from an open pin or from both pump-cell and reference-cell voltages exceeding 1.2 V. Its internal fault qualification uses passive and active stages totaling 2.5 seconds.

The continuation page describes a CJ136 alternative using hardware detection and a different internal qualification interval. It does not publish the same voltage-combination test for that variant. Applying the ATIC142 limits indiscriminately would go beyond the evidence.

Separate interface qualification from ECM counting

Once the interface reports a fault, the ECM uses 20 failures out of 24 samples at 25-millisecond intervals. It also requires ready/valid interface status, engine run or auto-stop, completed heater warm-up and more than 20 seconds of subsequent circuit-diagnostic delay. Listed heater and bank faults must be absent.

A voltage trace gathered before warm-up completion may not correspond to a running diagnostic. Preserve interface type and enable status when evaluating recurrence.

Verification should demonstrate an intact, correctly interpreted reference-ground signal path and a completed monitor without failure. P2251 does not establish the condition of a vehicle chassis ground from its title alone, nor does the source prescribe sensor replacement. The listed response is Type B, two trips.

Source and applicability

The documented system and calibration must be confirmed for the vehicle before using these limits. GM diagnostic table, PDF page 624. GM diagnostic table, PDF page 625.