P2628: NOx sensor 1 oxygen signal above its accepted ceiling

P2628 carries a pumping-current trim circuit-high title, but the cited GM decision checks the oxygen signal associated with NOx sensor 1. It fails when the reported oxygen value exceeds 29.00%.

Pressure conditions are part of validity

The engine must run with system voltage above 11 V and a fully operative sensor. Exhaust pressure must remain below 500 kPa, and the listed no-exhaust-brake qualification uses intake manifold pressure below 200 kPa. Invalid NOx sensor, pressure-related and specified manifold-pressure faults are exclusions.

The controller counts 100 failures out of 200 samples at a 25 ms task rate. Reporting is Type B, two trips. These gates make a reading under high backpressure or exhaust-brake operation a different diagnostic context from the qualified monitor.

What to compare

Retain oxygen percentage, pressure context and sensor validity together. The percentage ceiling cannot be translated directly into an external terminal-voltage limit, and an elevated NOx concentration is not the measured quantity in this test.

After diagnosis, confirm a completed upper-range check under the required pressure and operating conditions. A value observed only while the test is inhibited does not establish a pass. The summary explains the range decision but does not identify a specific failed wire, sensor element or pump-current adjustment.

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 1508.