Engine Coolant Sensor Check Engine Light

If you’ve noticed a check engine light on your P80 model (850, C70, S70 V70) and the ECM stores code 123 for the ECT sensor here’s your answer!

The engine coolant sensor (ECT) resides right underneath the thermostat on these vehicles and has been known to set a code for the ECT itself. The root cause is normally a thermostat that won’t fully close. The ECM looks for a 1 degree rise in temperature every 4 seconds as the vehicle goes through the warm up cycle, if this temperature increase is not seen code 123 is set.

When the thermostat does not fully close, some coolant slips past it instead of staying there to warm up with the engine. This moving coolant does not stay still long enough to heat up in the specified amount of time and the threshold is breached setting the code. Replacing the thermostat solves the problem, although most repair facilities will recommend the ECT as well for good measure.