The exact enable criteria for the P0161 is not clear. We do know that the PCM upon cold startup will run the diagnostic if: MAF is less than 27 g/s Engine runtime is greater than 2 seconds Coolant temp is less than 90°F Difference between IAT and CTS is less than 8°C As the PCM monitors the sensor upon cold start up, it expects the 450mv PCM reference to be pulled above 600mv or below 300mv within a time frame not specified. It is easy to pick out inoperative heaters, but heaters that are on the edge or slow can be difficult to pick out. From my experience, you are more likely to have one of these slow sensors set a heater DTC if they are post cat sensors due to the operation of the catalyst.

The location of the heated oxygen sensors and their wiring, many times makes taking amperage readings thru the sensors heater coil almost impossible. In that light, I approach these DTCs in this manner.

• Check for other history DTCs that may have caused the heater DTC to set improperly. The classic example here would be a poppet nozzle injector GM truck with a P030x DTC set along with the heater DTC.
• Check for engine performance conditions that may be affecting cold startup running.
• Let the vehicle cool down for 10 minutes and then with key on engine off, monitor the sensor(s) to see if it drops below 300mv or rises above 600mv within a few minutes (also compare to the other sensors).
• If sensor appears to be working and there are no other conditions that could cause a test failure, either replace the sensor (if under warranty or high mileage) or let the vehicle sit overnight and retest the heater. Normal heater coil resistance is from 3 to 10 W. The hotter the sensor, the higher the resistance, so the sensor heater circuits are going to be under the most stress when the sensor is dead cold. I have found an occasional sensor that will test fine until left overnight and then will be almost dead (takes a very very long time to warm up without the engine running)

Now, remember the rough idle and higher than normal idle emissions?

We are coming across quite a few of these. Like the CPI trucks and vans, these motors sure like to produce carbon! The techs downwind of me really hate it when I run GMs top engine cleaner through these…. One can usually smoothes them out and cleans up the tail pipe; occasionally, it takes two cans… So did I fix the P0161? Who knows…. It never came back so maybe yes, maybe no. I hate that! I really do. There are many straightforward easy OBD2 repairs, but more and more I see the tough ones. More to follow…

Home James