P2209: NOx heater resistance deviates from its target after several delays

P2209 is a performance check on the engine-out NOx sensor heater's resistance. GM's reviewed strategy compares actual heater resistance with target resistance using the normalized difference: actual minus target, divided by target. The fault boundaries are above +0.06 or below −0.06, equivalent to a deviation beyond 6%.

Timing is more than the sample counter

The monitor waits more than 45 seconds after supply is in range above 10.8 volts, more than 180 seconds after dewpoint is reached and more than five seconds after engine overrun. A combustion-mode enable also applies. These gates make a brief warm-up check insufficient to establish a heater-performance pass.

Once eligible, qualification uses 125 failures in 250 samples at 25 milliseconds each. The longer prerequisite delays should not be confused with that separate counter. Reporting is Type B over two trips.

Evidence that answers the monitor

Relay voltage must exceed 11 volts, the NOx bus relay must be on and communication and CAN data must be valid. The relevant evidence is target-versus-actual resistance after all gates are satisfied. A heater that merely draws current can still miss its resistance target.

Verification should confirm completion of the qualified performance test, rather than assuming recovery from an electrical continuity result. The row does not specify why resistance is off target or mandate sensor replacement. Exact hardware applicability and the permitted service test require vehicle-specific information beyond this 2025 GM monitor summary.

Source: GM monitor, PDF page 1414.