P2270: Rear oxygen sensor fails to respond to commanded enrichment

P2270 checks whether bank 1 sensor 2 can leave its lean voltage region during an intrusive enrichment test. In the reviewed GM strategy, it begins a sequence of six downstream oxygen-sensor monitors. It is not merely a judgment that one ordinary voltage reading looks lean.

Airflow determines the test opportunity

The controller increases delivered fuel while monitoring the downstream signal and accumulating airflow mass. The fault criterion is a sensor signal below 850 mV after accumulated airflow exceeds 70 grams during the test.

The 70-gram quantity is cumulative mass, not a mass-airflow rate of 70 g/s. Test duration therefore depends on how quickly air passes through the engine. A fixed waiting time cannot replace the actual monitored exposure.

Other faults can prevent a valid result

The source requires system voltage above 10.5 V and valid learned heater resistance. It also excludes fault groups involving airflow, pressure, temperature, injectors, fuel trim, misfire and the downstream sensor sequence. A failure in one of those areas can change whether this response test is available.

The normal frequency is once per trip, with specified fast-response exceptions allowing more tests. The row lists Type B, two-trip behavior.

A meaningful verification demonstrates that the rear sensor reaches the required rich response during an eligible commanded test. The code does not distinguish a sensor problem from every possible cause of an absent response. Another source occurrence on page 277 only references P2270 as part of catalyst-test sequencing; it is not this code's main diagnostic row.

Source and applicability

Use these facts only after confirming the documented system and calibration for the vehicle. GM diagnostic table, PDF page 628.