As a result of a discussion in this thread wether or not fault codes self cancel and recent experience, I thought I'd start a separate thread on this.

So, in that thread the question was raised wether or not you need to reset the stored error codes after fixing the cause.
As it happens I started getting error codes that do not self reset, and yet I have experienced the opposite in my previous 245.

What's going on?
The 1-1-3 my previous 245 threw would self reset by just turning the ignition off and on again.
The 2-3-1 and 2-2-1 I am experiencing since yesterday in my current 245 are more persistent.

I had erased both codes yesterday using the OBD1's reset function. Today I started the engine and within half a minute of idling the CEL lit again and both codes were back.
Hold on...the O2 sensor could not have heated up enough to give useful data within this very short time! Besides, it takes a lot longer for the coolant to heat up to the temp the ECU wants to see before it switches off the cold start program.

So, if I think about it, that would mean that the codes 2-3-1 and 2-2-1 read the "fuel trim" from the levels stored in memory by the self learning feature and not from actual realtime measurements. Those stored levels must have gone outside the permissible window and hence the codes were thrown again regardless.

To test that hypothesis, I performed a full reset by disconnecting the battery for a short while. I then drove the car and by the sluggish way the engine responded to the throttle, I could feel that all trim levels had been restored to factory default.

During that 10-20 mile trip, the CEL did not come back on.
Time will tell if the codes come back...

What are your thoughts on resetting error codes?

I'm not sure how the ECU handles codes reset through the OBD box. I've found it much quicker to erase all ECU memory by lifting the fuse (6 in the '92) for one second. This means you'll need to drive it for a few miles to relearn any trim changes, but eliminates any concerns about what is kept in memory. Losing power clears it immediately.
--
Art Benstein near Baltimore

Just found 2 lumps on my car battery.
Had them tested.
One came back positive.
Hope it's not terminal

The codes I tried to erase with the reset procedure returned within about 30 seconds. The hard reset I did next erased them permanently, and for now that was all that was needed.

My hypothesis is that the long term fuel trim went outside the permissible window, and clearing the DTCs did not change those trim values, so they returned quickly.
The hard reset cleared everything and with the cause being absent (for now) no CEL or DTCs have returned.

Hi,

Was the "Hard Reset" procedure the same as the one I put in a previous post for CoolVolvo below your new one?

https://www.brickboard.com/RWD/volvo/1677345/220/240/260/280/check_engine_code_212.html

I'm curious if that really works on our cars as well as the newer cars.

Phil

No I just disconnected the battery for a short while. The LH computers aren't particularly inclined to hold a charge for long.
Art's lifting of fuse 6 for one second is enough and does the same.

Ok, just checking the field of processes.
I agree our OBDs are not nearly as sophisticated or over lapped with BUS communication modules either.
If I owned a newer car I would keep this hard reset procedure in the back of my mind.
With what they charge to diagnose a car, it would kill me to know how easy that shop charge stabbed my wallet!

Phil

I remember finding error codes but couldn't find anything wrong at all with the car. So these must be old codes. After erasing the old codes via the OBD (not by disconnecting battery) and driving the car for a week, no more codes found afterwards. So I think codes don't self erase after repairs.

The CEL did self extinguish after repairs, after I replaced the O2 sensor last time. But the codes still remains in ECU memory unless manually erased.

Amarin.

The only time my 240 threw codes was when I crashed across a railway level crossing. I had one 240 owner in the car and another very experienced Volvo guy as well. We "limped" to our camp site a couple of miles further on and did some checking with code reader. Found some numbers but didn't have a check list. We disconnected the battery overnight and all was well in the morning when reconnected. Back to 1.1.1. We were going to leave it for 30 minutes and try again but went to the pub instead! Never had any further problems. 1992 model SE. We checked both of the readers, the main one on the battery side and the other small one on the other side. For me the battery disconnection fixed the problem so it might be the same for you.