Mazda 6 Forums UK

Technical Section => Heating & Ventilation => Topic started by: Danstine13 on September 16, 2021, 08:35:26 am

Title: Intermittent overheating (coolant) light on dash
Post by: Danstine13 on September 16, 2021, 08:35:26 am
Mazda 6 - 2.2 diesel - Manual - 2015 - Gen3 - SE-L

Hi all, as the subject line suggests - I am having an intermittent issue where I will be driving after half an hour or so on normal town like roads (including motorway) and suddenly the red temperature dash light will flash for a few seconds and the engine will go sluggish. The light will vanish after a few seconds and the car will run fine again.

I have used my OBDii adaptor and Torque and no error codes are stored. Nothing seems out of the ordinary. The coolant level is fine, no leaks, the quality of it I am unsure of, was replaced in a service 2 years ago (missed one due to Covid).

I am thinking possibly the following but would like others opinions either to validate my train of thought or to debunk it:

- Engine Coolant Temperature Sensor faulty (Torque seems to show it working okay but I haven't recreated the problem to see it in Torque) 
- Trapped air in the coolant system (although I think air can't just get in the system unless there has been a change)
- Thermostat faulty?

Also might be worth mentioning I have had a long-term issue with the auxiliary heat. When the engine has reached optimum temperature and I switch the auxiliary heater to hot, the heat only comes through if I accelerate. If I am idling, only cold comes through. This issue makes my cold mornings incredibly long and stressful! Perhaps these two issues are related in some way.

Any help would be greatly appreciated.

Title: Re: Intermittent overheating (coolant) light on dash
Post by: Atenzer on September 19, 2021, 08:07:19 pm
You haven't told us what mileage but you've pretty much listed the suspects for the cause.  However I can add another to your list, which is the water pump.  When the water pump is on its last legs, certain times the circulation of the coolant becomes insufficient which with the combination of driving at the time could get the temp high.  Equally reduced coolant flow means the heater won't work very well either unless revved.

However, it's easier to list things but in reality we got to balance the time and money.  If I were you, I'd start with replacing the thermostat because it's consumable and cheap.  At least one factor is eliminated from the list then.  If you could do a live reading from the coolant temp sensor by the OBDII kit you have, you could  then narrow down further.
Title: Re: Intermittent overheating (coolant) light on dash
Post by: Danstine13 on October 05, 2021, 04:31:09 pm
Thanks for the reply, it's 110k btw.

I'm going to change the thermostat or at least remove it and test it in hot water and then go on from there. Thansk for letting me know about the water pump, that also sounds quite probable.

I have also seen it could be a failed fan but the fans are very loud when idling so I assume they are working.

I assume changing the thermostat will require draining the coolant? From the service manual I see it isn't a top mounted thermostat.