Hi,

In the last couple of months I've noticed that the HVAC system on my Mk3 2017 Focus has not been coming up to temperature as fast as it used to. I recently replaced the battery, which revived the auto stop start on the car, and I hoped that would maybe have an effect on the heater behaviour, but no luck on this front. Arguably, it has gotten worse since then.

Subsequently, in the last month, the engine temperature guage has consistently refused to budge above the bottom of the dial. I've had drives from london to coventry where it hasnt moved a millimetre off the bottom. This typically happens if the ambient air temperature is ~1 degC. I assume that if the system thinks there is no engine temperature, then it cannot provide any heating to the HVAC system, thus no cabin heating for me.

The engine runs just fine, but the issue with the temperature readout does concern me. In previous years, in equally cold temperatures, I did not observe this behaviour on the temperature dial.

My only guess is that somehow the sensor itself is being overly impacted by the ambient air temperature conditions, enough to upset the readout given for the engine temperature.

Has anyone else experienced this behaviour and if so, what were the fixes?

Initial thought is that the thermostat may have stuck open. As a test have you tried leaving it running stationary for a while to see if the temperature comes up?

Bear in mind the gauge doesn't reflect the actual temperature. It's driven by the ECU (I think) so will typically only indicate normal temperature if the sensed value is within a certain range.

What engine is it?

Hopefully one of the experts will be along soon with better information.

No heat....thermostat or water pump but if everything else is ok then id say stuck thermostat.

On 2/22/2025 at 5:17 PM, mjt said:

Initial thought is that the thermostat may have stuck open. As a test have you tried leaving it running stationary for a while to see if the temperature comes up?

Bear in mind the gauge doesn't reflect the actual temperature. It's driven by the ECU (I think) so will typically only indicate normal temperature if the sensed value is within a certain range.

What engine is it?

Hopefully one of the experts will be along soon with better information.

I ran the engine stationary for ~10 minutes (ambient air temp ~8 degC) with the heater set to max heat. Temperature reading after maybe 8 minutes started to shift off the 60 degC (bottom) line of the dial but only barely, heat delivered to the cabin got lukewarm, but definitely not the heat level expected for max heat. Went for a short drive for ten mins after that, 20mph roads the whole way, still no change on the dial/heater temps.

Engine sounds like its running fine, no engine warning lights at all. Its a petrol 1.0L turbo ecoboost (120hp).

The experince of it getting worse since the battery replacement probably means a thermostat got knocked/damaged/whatever

Took it in to the workshop, turned out to be the secondary (electric) pump had failed. Replaced it and now back up and running fine!

 

In case anyone else sees this issue, the adivce to me initially was that it could have been a result of a coolant pipe that runs round the engine bay leaking out. Thankfully no such issues on that front for me.

I thought the electric pump was only intended to avoid the engine temperature rising after switching off. Are you saying it was actually running continuously and preventing the engine from reaching normal temperature?

If not I don't understand how it's failure could cause the symptoms you described.