Just a guess, but it might be cold related. The Wifeys Mazda 6 check engine light came on and stayed on. I scanned it and it was a weird "coolant colder than thermostat set temp" type code. I remembered it had been below freezing the week before and reset the code. Weeks later it has not returned.

I suspect there is a timer for measuring coolant temp and after the timer expired the coolant was still too cold for the programmed set point. I dunno, just glad it's gone.