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Should I fix a faulty Thermostat?

Our 2005 Mazda 3 appears to have a problem with the thermostat. My mechanic read a check engine light code connected with a thermostat problem. My wife has noticed that the car seems to take longer to reach normal operating temperature than it used to (close to 15 minutes of driving). Other than probably diminishing the gas mileage and having to wait longer for nice heat is there any down-side to this problem? Is it likely to get worse and fail altogether? The shop guesstimates it will cost around $150.00 to replace.

If the thermostat is indeed stuck open, it can’t get any worse and probably won’t do any damage. But you should still fix it. It’s wasting fuel and making your catalytic converter work a little harder than it needs to. It’s an inexpensive fix. Go ahead and do it.

Its in the early failure phase. It is closing up but not quite enough and it will get worse with time. Its a 2005 model so it isn’t far from its second coolant change and replacing the thermostat will dump most of the coolant out of the system.

You could wait until the second coolant change is actually due if you want, but I don’t see a good reason for doing that. I would recommend that you get the thermostat changed, a full cooling system drain and fill (NO FLUSH) and a new radiator cap. That should not cost much more than just the thermostat alone, maybe $10-15 more.

Get a couple quotes from reputable shops. If they say the word flush, leave and go to the next shop on the list. A flush adds unnecessary cost and damages the cooling system.

We actually had the coolant changed out about 4000 miles ago- so it shouldn’t need that done- maybe they can just top it off?

If your coolant was changed…you really should have gotten a new thermostat at that time. Did the shop mention this? It’s a very routine part to service as preventative maintenance since the consequences of a thermostat that is stuck closed are really bad. Most will fail open though.

Anyway - I would have the thermostat (and cap) done and the entire system doesn’t need to be drained for that. It will only go as low as the place where the thermostat mounts.

If your coolant was really should have gotten a new thermostat at that time.

Since when? Thermostat is not a scheduled maintenance item. You replace it when needed. I still have the original thermostat on my 05 4runner with over 250k miles - no problems what-so-ever.

I dunno, I used to change thermostats as a preventive measure every two years along with the coolant change and hose replacement. Just a lot easier to do all at once. The downside at this point though is the longer it takes to get to operating temp and closed loop operation, the more load you are putting on the catalytic converters. If you ruin one or more of these, they are $5-700 each, so a stitch in time . . .

Oy. Mike - it wasn’t a statement about what the manual probably says or anything REQUIRED. It was a statement about simple pragmatic preventative maintenance. And I don’t think I’ve ever asked a shop about servicing my coolant and NOT had them ask about the thermostat. On certain cars it might make no sense if the thermostat is very difficult to get to. But on most it is about as easy as changing an air filter.

“You” might replace a thermostat when needed. I think waiting until the thermostat is a problem is a bad decision.

"Other than probably diminishing the gas mileage and having to wait longer for nice heat is there any down-side to this problem?"

Yes, there is another downside to this situation that you should be aware of.
When an engine is run for longer than necessary at lower-than-normal operating temperatures, it will experience faster wear than an engine that is running at a normal temperature. No, the difference is not drastic, but if you intend to keep this car for the long-term, continuing to drive with a bad thermostat will lead to oil burning at a lower odometer mileage than if you replaced it promptly.

Also, your oil will build up contaminants much faster in the winter under these circumstances, due to the “richer” gas/air mixture that the engine is running on when it is colder than it should be. So, depending on your driving patterns, you may need to do your oil changes much sooner if you continue to drive with a bad thermostat. Otherwise, the engine will have a tendency to build up damaging sludge much sooner that it would otherwise.

$150 may sound like a lot of money to you, but–IMHO–spending this money now is cheaper in the long run, in order to avoid accelerated engine wear and the build-up of sludge.

You said your mechanic read a code. What exactly was the code that your mechanic read?

Probably P0125. Insufficient Coolant Temperature For Closed Loop Fuel Control.


Thanks everyone- I guess we will ask our shop to replace it and if they think we should replace the cap also we will do that too.

Probably, but everytime I ass-u-me…
I’d feel better knowing for sure.

Don’t know the exact code- It is not on the service invoice. Sorry.

Another downside beside reduced gas mileage is a potential clogged catalytic converter due to increased hydrocarbons, may be a $1200 repair.

One other downside of a stuck-open thermostat, the engine may not ever get hot enough to burn off water vapor that accumulates in the exhaust system. If the water vapor condenses, over time it will result in internal exhaust system rusting. Something best for your pocketbook to avoid. $150 to replace the thermostat seems a reasonable fee. I’d go for it.