Coolant Temperature Sensor?

[layn]

The coolant temperature sensor check engine code has been coming up on my 2004 XC90 T6 for awhile, on and off. Last week, a code was coming up for the thermostat so I replaced both over the weekend. Now, the thermostat code is gone but the coolant temp sensor is still coming up. The car is having trouble starting and idling strangely. Once I clear the code, car runs like normal and starts fine. Once the code comes back on, it’s back to running weird. It was running fine before replacing these parts, but fuel economy was pretty bad. Does anyone have any advice? I’m already 5k on a car I got for 5k a few years ago.

[Moose Test]

The poor starting and running coupled with poor fuel economy sounds logical given the error codes reported. If the ECM isn't able to accurately determine the actual engine temperature, then it's likely to be using failsafe values from the engine map. Enough to ensure that it can start and run, but perhaps way off optimal for the actual conditions.

If you've changed the sensor but are still getting a fault indication for it, that would suggest maybe that there's an electrical continuity problem between the ECM and the sensor, so check the state of the connections at both ends, and the wiring continuity between. Other than that, it's also possible that the ECM has developed an internal fault and can't read the sensor.