Bought my Miata, driving it home from the NY State DMV with a Temp plate through Queens and my damn check engine light comes on. Stopped by AutoZone before heading home and its P0126 code. The system is running cool. A little research on the forums and it’s a common problem: stuck open thermostat.
Quickly bought a Stant 48728 thermostat with a decent discount from Advance Auto Parts for about $22 and then waited for months before doing anything about the fix. Basically waited out the winter until it was warm enough in the spring to do the repair. So now it’s April, I kept reading the forums and people recommended replacing the overflow tank at the same time, and instead of using Mazda/Ford’s FL22 green coolant, it would be cheaper to buy Zerex Asian Coolant which is pink. $58 for the overflow tank from TASCA parts and $28 for two jugs of Zerex and done… four hour job doing it slowly.
Original blog post: http://myroadtrip.net/mazda-mx-5-repair-lazy-thermostat/