Some vehicles may experience Check Engine Light illumination with a trouble code indicating the thermostat is stuck open. The thermostat will need to be replaced with a modified one and there is a software update for the Powertrain Control Module (PCM).Google+
Car Problem Reports
Mazda Mazda5 Check Engine Light/Trouble Code Due to Stuck Open Thermostat
Mazda Mazda5 Problem
Average Mileage: 72,392 mi (26,383 mi - 101,000 mi)
Visitor, 2006 Mazda Mazda5, 56,000 mi
po126 engine code came up...insufficient coolant temperature for stable operation. Mazda shop said i need to change my coolant thermostat. They charge $100 bucks for the part, but you can find it for less than $50 at online stores.
Visitor, 2006 Mazda Mazda5, 70,652 mi
Check engine light came on. My OBD-II scanner detected code P0126, "Insufficient Coolant Temperature for Stable Operation". Radiator fan was not running at the time of the problem. Turning on A/C, however, did engage radiator fan. Therefore to be safe, I set A/C to 90F with defroster fan manually set to MAX and proceeded to nearest Mazda dealership. On the way there, real-time OBD-II diagnostics indicated coolant temps between 79 to 85C in mix city/highway driving and an outdoor temp of 72F. Unfortunately I didn't have a baseline to compare or know what temps were considered normal. Dealership charged $90 for diagnostics; said the coolant thermostat was to blame. They also recommended a coolant flush, to which I reluctantly agreed. Total bill: $536.
This all happened on a family vacation in an unfamiliar place and with no Internet to do any research. Don't get screwed like I did.
Visitor, 2007 Mazda Mazda5, 26,383 mi
Dealer tested and found stuck open thermostat and quoted repair price of $399.97!
I said I will wait until after Christmas, he said watch temp gauge. Naturally extended
warranty had just expired, go figure!
Visitor, 2006 Mazda Mazda5, 71,000 mi
Check Engine Light on - Code P0126
Visitor, 2007 Mazda Mazda5, 70,000 mi
Thermostat was replaced
on the trip HOME ( when we bought the mazda) the check engine light came on...and stays on. the most scary thing is the rattling noise in the front end... that the dealer SWORE this car was checked top to bottom for re-sale... whoo hoo what a MIS-REPRESENTATION THAT WAS !!!!!
Visitor, 2006 Mazda Mazda5, 83,337 mi
Engine light came on, we found nothing wrong. Took car to dealership, and thermostat stuck open. Couple hundred to fix. I've also had some issues with stalling when slightly accelerating. I re-guested that they look at the date of the PCM and to update it.
Visitor, 2007 Mazda Mazda5, 69,000 mi
Same code, same diagnosis. Stuck open thermostat. Thermostats wear out. $282 to fix it. If I were savvier, I would replace it myself.
Visitor, 2006 Mazda Mazda5, 91,000 mi
I just replaced my transmission a two weeks ago and I was driving yesterday and my check engine light went. I took my car to my mechanic and they told me it was my thermostat...and it was going to cost $200 to $300 to fix. What the heck, I have spent almost four thousand dollars on this car in this past two months.
Visitor, 2006 Mazda Mazda5, 95,000 mi
check engine light on, replaced thermostat myself, engine light went off.
Thermostat was stuck open, but just a tiny bit. You could run tap water through it at a dribble over the sink.