P0126 After New ECT Sensor & Thermostat

Backstory:

Lately I've been seeing weird fluctuations in my ECTs which prompted me to replace my ECT sensor. Once that was replaced I thought all was fine until one night I noticed my coolant temps drop to 150-160 and then in the drive through after some time had passed they soared up to 244 before starting to drop slowly as I babied the car home.

At that time the car did leak coolant and it's safe to say something mechanical went wrong.​

Since then I've replaced the thermostat and while it's only been a few days, I'm still getting datalogs like this:


Parts replaced:

OEM Mazda engine: ~28K miles
Radiator cap: February 2022
Coolant hoses: December 2022
ECT sensor: March 2024
Thermostat: April 2024
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Coolant System Pressure Test:

I picked up a cooling system pressure tester off Amazon and followed the instructions, pumping it to 20psi and then watching for the needle to drop. To my dismay, it did drop, albeit slowly. In around 2 minutes and 50 seconds, it dropped from 20psi to 15psi, about 1.77psi/min drop. During this test I was not able to hear any obvious leaks, nor did I see any coolant spraying anywhere.

I also checked the cylinders by taking out the spark plugs and looking inside, again nothing obvious and they all looked fine and dry. The sparkplugs also looked decent.
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Coolant System Block Test:

In addition to a pressure test I also did a block test with the fluid that detects combustion gasses in the fluid. This did not show any signs of gasses in the coolant thankfully, and the fluid remained blue after 1 minute of aeration.​

So I'm left here scratching my head wondering what to do next. My thinking is the radiator cap and then possibly another thermostat. You just never know with these cars and the parts it uses. Worst case I suspect something wrong with the headgasket or head, but it could also be some gremlin with the electrical system, to which I haven't fiddled with yet.

My main goal is to prevent another engine being blown, so I'm hoping for some next steps from the wizards on these forums.

 

Here are some workshop manual resources that may prove helpful to others in a similar situation.

1. DTC P0126 Workshop Manual
   
2. Cooling System Pressure Test Workshop Manual

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Here is a video of the issue as well.

 

Did you use all OEM Mazda parts?

Seems a little ridiculous, but sometimes other than OEM parts don't function like they should on these cars.

 

Everything was OEM except for the hoses, to which I got a silicone hose kit from Mishimoto.

 

What did you do to the car between the last time it didn't do this and when it started? Can you revert back and potentially correct the issue that way?