Old car, new problem.
My old Volvo has decided that I've had enough fault free motoring for the time being and has elected to start throwing random engine fault codes. The first was code 122 which is the IAT sensor. Code 231 was also stored, but I thought that the IAT might have triggered that. Luckily I had a spare IAT so swapped it in, forced a reset and all was good for about 100 miles. Then the check engine light is back on though with no loss of performance. I pulled the code and this time it was just 231 which is fuel trim too rich. Looking online there are differing opinions as to what might cause this.
I reset the light after checking for vacuum leaks and found none. All well for another 100 miles then light is back on with the same 231 code. I've dismantled all the air and vacuum lines that I could, checking for splits and leaks and have found none. I reasembled carefully, smearing a little oil at every join before tightening everything back up hoping that this will solve any minor leaks. I took it for a test drive giving it some beans and all is well. I cant shake the feeling that it is only a matter of time before 231 comes back
Diagnosis of what these codes really mean is tough. I eventually tracked 231 translated into real world speak being a mismatch between what the exhaust O2 sensor is reading and what the ECU is expecting based on the MAP (Manifold Absolute Pressure) and IAT (Inlet Air Temperature) sensors. This is often apparently caused by a vacuum leak, which I can find no trace of. However, sometimes other things can cause it, but it was unclear as to exactly what.
Does anyone have any experience of these codes? Every time they were tripped I was doing 70* on the motorway. (*yes officer, bang on 70 and not an mph over though if you tell me I was doing faster I'll need to have the speedo recalibrated at the soonest opportunity, thank you for bringing it to my attention). I cannot replicate them just by running the engine. Would driving a vehicle hard show up a minor vacuum leak that might not be present during gentler running? I shall try driving slower at a steady 60 for now and see whether it comes back. I am also picking up a new air filter because it has been very dusty these last few months and I wonder if dust and pollen has clogged it though it looks in reasonable condition. I'm also changing the plugs because whilst they look fine and show nothing unusual they have been on the car for around 60,000 miles.
Seeing the check engine light come on is very annoying, as I checked and the engine was definitely still there so what more does it want? Any knowledge passed on would be gratefully received.