All: I've been dealing with the appearance of these check engine codes for a while on my 1990 760T that I only drive in the summer. I've put on under 1,000km this year since May. I've also read the prior posts related to issues with these codes - which are many. Current mileage is about 116,000km. Each time I've driven short distances (e.g. under 50km) the check engine light has been coming on. Ive then pulled codes and disconnected battery to clear.

Here is a summary of the codes from this past weekend.

- I ran some errands in the city for about 75km. It started and ran well and the light stayed off. I was down to about 1/3 tank which still had some fuel from storage (94 octane with stabilizer) and refuelled to full with 94 octane. During the last kilometer, at lower revs (not sure if this is an issue or not), the light came on just before arriving home. At idle at home, the exhaust was noticeably a bit dark (rich I presume). I pulled the codes and got 2-3-2 and 1-1-3.

- I reset the codes (no battery disconnect) last night, and drove at highway speeds (110-120km/hour) for abouty 150km this morning. The light stayed off and the car ran well, perhaps slightly rough low idle but no stalling or coughs.

- O2 sensor was replaced at about 114,000km, and I replaced the cold start valve at about 115,000km. at suggestion of knowledgeable Volvo mechanic.

I'm wondering now if this could still be due to a fouled O2 sensor, or possibly something else like the fuel pressure regulator (?). I have not had time yet to check voltages as suggested in a number of other posts. Would driving at highway speeds (higher revs, higher engine temp) have anything to do with the light staying off?

Any ideas on root cause and best path to trouble-shoot and fix are greatly appreciated. Regards.