P2627: NOx sensor 1 oxygen signal below its accepted floor

P2627 is generically called oxygen-sensor pumping-current trim circuit low. The GM row implements it as an oxygen signal-range check associated with NOx sensor 1. The documented lower boundary is −6.00% oxygen.

What is actually too low

The threshold is a signal-validity floor, not a physical claim that exhaust can contain negative oxygen. It is also not a published pump-current or connector-voltage threshold. A low NOx ppm reading is a different parameter and cannot establish this oxygen-percent failure.

The engine must run, system voltage must exceed 11 V, the sensor must be fully operative and the applicable combustion mode must permit the test. An invalid sensor-1 condition is excluded. Reporting uses 100 failed observations out of 200 on a 25 ms task, with Type B, two trips.

Diagnostic interpretation

Identify the oxygen parameter associated with NOx sensor 1 rather than substituting a conventional narrowband oxygen-sensor voltage. Record its validity state alongside the raw or appropriately scaled concentration. An unready sensor cannot demonstrate a qualified range test.

Verification should establish completion of the lower-range monitor after diagnosis. The code does not itself prove a failed pump-current circuit component or provide an adjustment procedure. Vehicle-specific information is required for any electrical test, sensor architecture or repair decision.

Source and application limits

This explanation covers the cited 2025 GM monitor. Confirm the vehicle and calibration in its service information before applying these limits; inclusion in this file does not establish model compatibility. GM diagnostic parameters, PDF page 1507.