('07 Cobalt LS, 2.2L Ecotec)

In this car's 'mode 6' OBD2 data, there is a GM-defined item, "Post Catalyst Sensor Open Test" (Test ID 8A). This page shows it as a count. My scan tool also shows it having an accompanying "Maximum" (480). Whenever the count exceeds this Max, the test is marked as "Failed"; otherwise, it can be "Pass", or (occasionally) "Incomplete". The count seems to be reset to 0 at startup.

Over ~150 trips I've looked at, this count has ranged from 0 to 800. If over 480, it's flagged as "Failed". I have replace the sensor in question, to no effect.

Anyway, questions

  • What triggers the count? Is it literally an "open" circuit, loss of continuity?
  • Is it testing the O2 sensor's heater, or the sensor itself?
  • What interval are the counts accumulated over?
  • Can a "failed" status lead to a CEL/MIL? (I've had no CEL codes explicitly indicating an Oxygen Sensor failure.)
  • Can a "failed" status = without a CEL - prevent the EVAP test from running?

1 Answer 1


Open generally means that the circuit is open, so a loss of continuity. If you have replaced the sensor then it's probably an electrical problem in the wiring. It could be a problem in the ECU itself but it's more likely something in the connector or the wiring loom. I'd start by checking the connector for faults and giving it a good clean, then work your way up to see if there's chafing or some other visible problem.

Open heater circuits do have a separate diagnostic code (P0135), whether GM uses that code or lumps continuity together with the sensor I don't know. I'd assume they don't for now. The troubleshooting is the same process anyhow.

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