Hello. Had my Focus 2.0 Auto, 2006 for two years now and it has been completely trouble free. It still runs perfectly, but it has suddenly developed a "fault" associated with odd readings on the temperature gauge, whereby it appears to run hotter than normal (though not overheating) and also the air-con fan stays on even after the A/C has been switched off, and stays on until the engine is switched off. Up until now the engine has warmed up within a very short time, and the needle has always sat on the first quarter mark of the gauge (75 degrees). It would only ever get hotter when stuck in traffic etc, at which time the temperature slowly climbed to just before the half mark (90 degrees), when the cooling fan cut in and the temperature returned tto the quarter (75 degrees) mark. This would happen all day, and using the A/C or not made no difference.

Now, the engine warms up as normal, and the needle sits on the first quarter mark of the gauge (75 degrees) as normal for say 20 minutes / 10 miles? or so, but then the temperature rises to the half mark (90 degrees) fairly quickly, and stays there. It will come down again if you keep the car going at some speed, presumably to get the max "ram air" effect. I have considered a problem with the thermostat, but from reading many comments on various forums I am thinking the problem lies somewhere with the engine cooling fan, or whatever controls it, especially with it remaining on once the A/C is switched off. What I can't work out though is how the reading on the gauge fairly quickly goes from the quarter to the half mark, indicating a fairly rapid increase in engine temperature? and I cannot see what this has to do with the cooling fan?  Any ideas / comments appreciated!!.

9 hours ago, UnSol said:

the problem lies somewhere with the engine cooling fan, or whatever controls it,

Usually there is just one coolant temperature sensor, mounted in or near the cylinder head. This is connected to the PCM (engine computer), and the PCM uses it to control the fan speed, via a single control wire to the fan motor controller, which is part of the fan. This unit also receives a battery supply to power the fan. Since the fan does run, it must be getting power.

The information from this temperature sensor goes over the car CAN bus to the IC (Instrument Cluster), where that unit drives the indicator gauge.

The indicator gauges are sometimes very coarse, just designed to indicate cold - normal - hot really. So can not be entirely trusted, and may jump about a bit for quite small temperature changes.

Without some separate measurement of engine temperature, it is hard to separate fan & sensor & thermostat problems.

If the upper radiator hose stays moderately cool as the engine warms up, then heats rapidly once it reaches thermostat temperature, then the stat should be ok.

The fan will run on if the sensor is indicating high temperature. If this happens when the engine is actually at normal temperature (80C), then the sensor is likely to be faulty.

Hello Peter,

Many thanks for your response to my recent post. Your information has been very helpful with my attempts at understanding the problem(s). It would appear the main suspicion of a fault is with the coolant temperature sensor, on the basis that a fault (as far as I can gather) with the PCM is relatively rare. A fault with the coolant temperature sensor would also account for both the odd cooling fan operation, and the change in readings on the temperature gauge.

Thanks once again. I will come back to this forum and report in due course, hopefully with positive news!

Update. Problem solved..!! Took the car to a local (and trusted) mechanic whose first suggestion was a problem with the thermostat, given the "odd" readings on the temp gauge. Apparently, thermostat failure is quite common on the Focus. He also suggested that the gauge should normally read just before (or on) the "straight up" 90 degrees mark, whereas since I have had the car it has always read on the first quarter mark of the gauge (75 degrees). So, one new thermostat and housing later all is well, with the gauge quickly getting up to the (just before) "straight up" 90 degrees mark, where it stays without any movement at all..!!! And with the thermostat replaced, the fan is now working correctly.