Both coolant temp sensors are reading accurately according to the resistance test in the Mazda service manual and voltage displayed in the PIDs. They fall right in spec in the graph of temp vs kilo ohms when I checked the sensor with a infrared thermometer. But this coolant monitor PID shows -40F.

What's the problem and how is it seeing that temp if the primary and secondary sensors see normal operating temp and both the same?

 

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A quick Internet search shows that this is not uncommon.
Mazda has no documented calibration process for these specific sensors.
I would hazard a guess: if your infrared thermometer is correct, your sensors are toast.

 

I deleted my response because the OP needs to do more (lots more) research and study about how the testing was concluded and what the information on the devise was showing.

 

I'm back after learning as much as I could about testing the ECT sensors. I even have scanner danners diagnostic book and went through the testing procedure not only from his book but from the Mazda factory service manual. He responded on his YouTube channel and had my back probe the connectors with the key on engine off and then remove the connectors to see if the signal changes to 5v reference. Also checked the grounds. Everything is good. The sensors test perfectly. They respond accordingly with the chart in the service manual. They read perfectly in the scan tool. There is no codes present. The engine runs normally.

Then I replaced the sensors just in case. After all, they have 175k miles on them. Zero difference in their readings.

So, please help me understand why the scan tool shows -40F for Thermostat Monitor Engine Coolant Temperature Max Limit, Min limit and Temperature.

 

I also noticed the injector pulse width is way higher than normal. Hundreds of times more. Yet, these are the only PIDs that are out of the normal range. There's no way the injectors are staying open more than a second, right? How would it run? And then the fuel trims would be maxed out and they're not.

 

• The -40°F reading typically occurs when the engine coolant temperature (ECT) sensor is disconnected
• This value is a common placeholder for an unplugged or malfunctioning ECT sensor.
• Essentially, it indicates that the sensor is not providing any data to the engine control module (PCM).

 

I'm aware of that but that would apply not to the thermostat temp monitor, but to the ECT sensors themselves. They do not show -40. They read properly. Both of them. So either there is another sensor that I'm unaware of or there is something else going on here. Why would the monitor PID show -40 when neither of the two sensor PIDs do?

 

Have you tried using a different scanner. Not a blue tooth one but a Professional grade connected directly to the diagnostic connector.

I would recommend not to change any part regardless of miles or age unless they are confirmed at fault!

 

I have not. This is a topdon Phoenix elite. It's a $900 scan tool but it could be wrong. It's the only one I have. I've got a cheap Bluetooth code reader and an innova code reader with a physical cable but I don't think either of those will read these PIDs.