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I've got a 2008 Mazda 3i Touring, 5 speed manual, with about 145k miles on it. A few weeks ago, after my wife got home from work, we noticed the coolant temp gauge on the dash was fully at high and there was a slight burning smell. I don't know how long the gauge had been at high, but her commute is about 10 miles in a mix of stop-and-go and highway traffic.

I checked the coolant reservoir (the front passenger side of the engine compartment, right behind the washer fluid reservoir) and it was low. I put in about 1/2 gallon of coolant (premixed), and haven't seen any leaking out since.

At the same time, the check engine light has been on (first on since April). Back in April I had an auto store read the code, and it showed P0446. I didn't do anything to address that at the time, and the auto store didn't clear the code or anything.

Even with the additional coolant, the temp gauge will creep up towards high while driving, but it tends to go back down (to 1/2 or 3/4 of the way to high) as rpm and speed fluctuate. I don't drive it far now, usually only a few miles each way and only a couple times a week.

I had the auto store read the engine codes again today, and this time there were 4. P0446 showed up twice, along with P0117 and P0126. I also noticed a bit of a burning smell from the engine area after I got home.

The last two codes seem to be coolant related, based on my quick Google search, but are they a cause or a symptom? What could be causing my coolant to continue running hot?

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  • The cooling fan may be worn out, fuse blown or faulty fan relay. If your ac works, turn it on; the cooling fan always runs with a working ac system. If the fan runs or seems slow, it may be worn out or dead if it doesn't run at all. Ac demands cooling fan operation because compressor operation creates hot refrigerant sent to the condenser coil in front of the radiator, hence fan operation to force airflow thru two radiators otherwise a worn out or dead fan allows the coolant to overheat. If ac is broken, you can apply 12v power directly to the cooling fan to test it.
    – F Dryer
    Commented Oct 17, 2023 at 4:47
  • @FDryer AC system seems to have a refrigerant leak, too, as it was not blowing cold air all the time (no need for much AC now).
    – mmathis
    Commented Oct 17, 2023 at 13:08
  • A P0446 error code indicates an evaporative emission control system (EVAP) vent control circuit malfunction. Simply put, the computer has detected an issue regarding the functionality of your vehicle’s EVAP system vent valve.
    – F Dryer
    Commented Oct 17, 2023 at 21:21

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There's a few places we can go with this. The codes you pulled are reporting that on one side of the coin the thermostat is stuck open and engine is not warming up fast enough. The other codes says the engine is overheating. These codes may or may not be accurate. On one trip the thermostat could have gotten stuck closed and caused it to overheat. So there is a possibility that the thermostat needs replaced. This may or may not have caused it to overheat. The coolant level dropped, so you need to find out why. Did it leak out and cause it to overheat? Or did the thermostat get stuck, causing it to overheat and eventually blowing coolant out the pressure release valve? Now that it overheated and lost coolant and coolant was replaced, you may now have air in the system.

What needs to be done is to make sure coolant is filled and air is bled out of the system. Then a pressure test should be performed. If it passes the pressure test, the thermostat should be changed whether it was faulty or not. Then test the coolant sensor or just replace it. Then check to see if the sensor is reporting properly. If not repair wiring.

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With a dead ac system, testing cooling fan operation; a cooling fan relay switches power to the fan from ecm commands and may be bypassed with a jumper wire across the relay socket terminals or simply connecting the fan directly to the battery. In either case, the fan should power up immediately. If the fan has more than two wires, you'll have to determine which wires are used for high speed and connect to battery. Other fan circuits use two cooling fans with several relays to control low, medium and high speed with resistors - more complicated wiring requiring wiring diagrams. You decide which way to proceed as you may have more than one problem.

A P0446 error code indicates an evaporative emission control system (EVAP) vent control circuit malfunction. Simply put, the computer has detected an issue regarding the functionality of your vehicle’s EVAP system vent valve.

Diagnostic trouble code (DTC) P0117 stands for “Engine Coolant Temperature Sensor 1 Circuit Low.” It indicates that the vehicle's powertrain control module or PCM has detected a low voltage input from the engine coolant temperature (ECT) sensor 1. Examine the coolant sensor and connections. A faulty sensor or damaged wiring/connectors may not send correct signals to the ecm.

Diagnostic trouble code (DTC) P0126 stands for “Insufficient Coolant Temperature for Stable Operation.” This code sets when the engine does not reach normal operating temperature within a specific time period. When the engine reaches its specified operating temperature range, it will be able to function optimally.

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