Volvo C70 1998-2005 » Engine Coolant Sensor

If you’ve noticed a check engine light on your 850 or 70 series Volvo and the ECM stores code 123 for the engine coolant sensor (ECT), here’s your answer! The ECT resides right underneath the thermostat on these vehicles which 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 warming 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. If your vehicle happens to be running high performance parts, especially turbo models, consider using a colder thermostat to help counteract the added heat load.