Help troubleshooting coolant temp and codes

Post by **hausmeister** » 11 Jul 2025, 12:19

Hello, need some advice. Volvo 850 T5R 1995

I had some intermittent issues that the coolant temp gauge would just cut out (go to 0). I would restart the car and it was back.
Then some months back I fixed the ECT sensor connector + wiring, which was in a bad state. Since then I had no more cutting out.
I have not driven it much, but since then it has this strange issue that the coolant temp gauge drops as soon as I accelerate somewhat, and a lot when you floor it.
But there were no drivability issues, until now - the (not original) motronic 4.4 limits it to half boost and it has set the following codes:
EFI 232 (Long term Idle air trim)
EFI 123 (ECT)
I hooked up my scanner and while driving coolant temp drops from 100°C/210F to ~74°/160F unter acceleration and takes almost a minute to go back up.

Should i replace the thermostat, or the sensor? both? Not sure what to make of the idle air trim.

Can the Thermostat cause such a drop in temperature? Which would mean the sensor is working correctly.
(I flushed the cooling system multiple times last year and believe it is working as it should otherwise)

aeg · Post by **aeg** » 11 Jul 2025, 16:23

The thermostat could be stuck open. I would replace it. Be careful of the bolts on the thermostat housing, they strip easily and can be difficult to remove.

Vova585 · Post by **Vova585** » 11 Jul 2025, 16:26

Sensor is a cheap part to try. Same is true for thermostat. Both are reasonably easy to replace for the person that performed multiple flushes of coolant. I would opt to buy quality parts(oem preferably or OE). You can always play with resistance checks while "boiling your sensor) or watching thermostat open/getting stuck by similar submerging into hot water, but i believe your summer is more important than such old fashioned tricks

Vova585 · Post by **Vova585** » 11 Jul 2025, 16:28

AEG totally right about those bolts. Make sure they are clean and little tap "star" bit to "shake" the old bolt.
Until you will not correct your coolant temp issues, I would not warry about other codes

Post by **hausmeister** » 12 Jul 2025, 01:08

Will do, thx. Wouldn't say it is easy for me, but I have done it before.

I just wonder why it reports the temp drop. It warms up normal/fast, and otherwise keeps the correct temperature, and the sensor is reporting it. I'll replace the parts, just trying to understand it.

Post by **hausmeister** » 12 Jul 2025, 06:24

Just replaced the thermostat as I had a spare, but did not help. It looks like it was again the connector, the pins were pushing out on the back. The replacement parts are not the same as oem qualiy... Probably would be best to use a different connector type.

I wrapped a bit of wire around one of the cripms and soldered it, so it cannot easily back out of the rubber boot again.

Post by **abscate** » 13 Jul 2025, 04:37

I would have gotten that wrong too. Everything pointed to a bad thermostat. Bad wiring is tougher to diagnose when it is just adding resistance vs an open, or a short