Another Mazda3 P0126
 

2004 Mazda, Mazda3, 5dr Hatchback (2.3L 4cyl)

A week or so ago the Instrument panel "Check engine light" came on.
I took it to Advance and Autozone, had the code read and both times got P0126: "insufficient coolant temperature for stable operation". The hydrometer tells me my antifreeze concentration is providing sufficient degree of freeze protection and later research revealed that thermostat failure is a very common problem with first Generation Mazda 3's. I assumed this (thermostat failure) was my problem as well and planned to replace it.

The day it first occurred the outdoor temperatures were below zero: as low as -10. This week temperatures got way up into the 30's. I noticed the temperature gauge is right in center and I'm getting heat. I temporarily disconnected the battery to reset. I've only driven it a few times since but so far, the CEL has not yet returned.

Is it possible the extreme cold weather overcooled the engine dropping it below a normal threshold, fooling it into thinking there was a mechanical problem (when there really wasn't) that would cause it to throw the code?

If the thermostat were shot (stuck open) the temperature gauge couldn't read as normal and I'd be unable to get heat in the passenger compartment. My understanding has always been that a thermostat is either OK or broken (stuck open or closed depending on the make/design)..but once it's gone, it can't come back. Can they ever be sticky, behave erratically? OK one day, not the next?

I've read others' posts experiencing similar problems in this forum but found no conclusions.

When reset but with an existing problem, how many driving cycles are required before the CEL/warning returns?

Thanks for any assistance.

I can't answer all your questions, but I try to touch on a few.
If you replace the thermostat make sure you get a high quality one.
A thermostat is essentially a mechanical devise that can fail in more than one mode: stuck open, stuck closed and stuck somewhere in between.
The extreme cold could have had an effect on how fast the engine warms up, in particular if there is or was a minor air bubble stuck in the system.
I don't know how many cycles it takes for that CEL to return but it typically takes two subsequent failures of the same kind to trigger the CEL.

Interesting that you mentioned this; I just posted about this as well.
What exactly do you consider a "high quality one" since everyone warns against using third party replacements and it's the original Mazda thermostats that seem defective and fail in the first place?