P23B2: NOx sensor 3 heater control is shorted low

The documented P23B2 monitor checks NOx sensor 3's heater-control pin for a short to ground. “Low” refers to this electrical classification, not the sensor's NOx concentration or heater target temperature.

Operating conditions explain timing

The sensor must have reached dewpoint. Its bus relay must be on and supply above 9.90 V, with powertrain relay voltage above 11 V. Lost communication and invalid CAN data are exclusions. An 80-of-160 counter runs on a 25 ms task, with Type B, two-trip reporting.

The dewpoint requirement is particularly useful when a fault returns after warm-up but not immediately at key-on. That timing can reflect monitor qualification. It does not, without additional evidence, establish a heat-sensitive component defect.

Keeping the circuit identity clear

P23B2 concerns the heater-control path. The separate heater-sense low-voltage entry is P23B8; testing only the sensing path would not answer this code's circuit question. Obtain the vehicle-specific diagram and preserve the sensor's electrical report before any intervention.

A meaningful confirmation shows that communication remains valid and the enabled heater-control monitor no longer detects the ground-short state. The GM summary provides no pin number or universal voltage measurement, and it does not prove that replacement of the sensor assembly is necessary.

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 1457.