An O2 sensor heater circuit fault can trigger a MIL in a single trip. Unlike other systems such as the evaporative emissions, or catalyst which require two fail trips to light the MIL. So it's likely that you did repair the vehicle.

Next time when faced with an O2 heater. Test for power to the sensor heater circuit, key on, engine off, and then test for power to it key on, engine running. Different manufacturers use varying strategies, so you have to test both ways. Once power to it is confirmed, then test for power coming back out of the sensor. First you will only see p[ower coming back out of the heater circuit "IF" the hbeater is good, and the computer is not grounding it at the time you are testing. To find out if the computer is attempting to control the heater circuit, unplug the sesnor, and connect a testlight from the power connection of the vehicle harness to the ground connection for the heater. You will get one of the following. No light, which could be the computer is not turning the sensor heater on, or cannot turn the heater on because of a computer or wiring fault. You could have a steady or a blinking lamp indicating the computer is turning the heater on successfully.

This is a quick overview. Proper understanding of the circuit, and it's operation combined with proper testing take all of the guess work out of fixing cars. One test I left out is actually measuring the heater circuit current draw, and commanding the computer to turn the heater on to allow that to happen with a scan tool.

Thanks a lot, John; I really appreciate the detail you provided. My service manual for the truck is very vague on a lot of this, so your response points out the need to get a book just on engine electronics/OBD II and learn a lot more than what I know.

Your welcome.

BTW, get used to vague service information. Most aftermarket versions of repair information is watered down about 50% of what is available as compared to MOD (Mitchell On Demand) or Alldata. From there even they have gaps for varying reasons that can be filled in by going to the full source, which are the O.E. webages that can be accessed by subscription. However even with all of the service information that is out out there, the one thing you cannot get with it is training on how to use it and how the things that are on the cars really work. That takes a lot of studying on your own, first hand experience gained over a few decades, and constant classroom training.

To reset the service / fault computer on my Opel Astra, without inserting the key in the ignition, I just press and hold the reset button on the trip computer for five seconds. As Opel are the European part of the GM group I guess they have the same computer on all their models.

If the fault persists, the code will come up again.

You will not find this in the owner's handbook.