I originally posted this thread
(https://volvoforums.com/forum/volvo-...14/#post391978)
trying to resolve an engine warning light problem which I thought was cat converter/lambda sensor focussed, but after extensive work/replacement have now identified it appears to be an engine coolant sensor problem, so have updated my original thread and started this new thread to focus on this.
A couple of months back I bought a reasonably clean 1992 model 240 Torslanda 2.0 Estate ( 167000 miles on the clock) at a good price because it had failed its MOT on emissions and been off the road for about a year.


Changed cat and lambda but although had obviously been running very rich with heavy carbon deposits the engine warning light remained on.


However, had an epiphany moment and discovered LED diagnostic box which I had no idea existed.
This had a number of memorised alarms but after reset it only identifies an engine coolant sensor problem, so replaced it (not expensive although a pig to access) but no change.


Then checked wiring and plug/socket connectors (using digital meter) which appear good with continuity from section to section and end to end (one side of sensor plug to ECU plug mounted in passenger compartment on RHS of firewall; other side of sensor plug to a Bosch control unit mounted in passenger compartment on LHS of firewall which I assume is for injection/ignition control but can't identify it referenced anywhere).
Even bypassed wiring (bit of a Heath Robinson lash up with crocodile clips, etc, but good end to end continuity) with no improvement.


Then measured cold and hot sensor resistances which seem OK (cold about 2600 Ohm at ambient 26 degrees F and hot 270 Ohm at about 180 degrees F).
During this process disconnected sensor plug with engine running and it does not affect anything (engine continues running at same speed, warning light still on, same LED readout) so obviously there is an open circuit somewhere which causes the system to run fuel rich all the time.


My final check of the day was to measure voltage either side of sensor. This was just under 6v which on a 12 v system doesn't seem right. If too low it could be the problem, but as I don't have a wiring schematic can't be sure!


Gave up for the day and retired to put on my thinking cap.
My next actions will be to try to access a wiring schematic and to remove the dashboard facia panels to get access to the concealed wiring and check that there is nothing happening here (intermediate plugs, immobiliser/ignition switches, etc).
Any input/suggestions would be much appreciated.