Check Engine Light/Trouble Code Due to Stuck Open Thermostat on Mazda Mazda5

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).

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Average mileage: 82,365 (26,383–205,000)
5 model years affected: 2006, 2007, 2008, 2012, more2014
36 people reported this problem
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Viewing 1 - 20 of 23 problem details
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.
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.
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.
Same problem, but the light goes on and off depending on the week- very annoying!
2006 Mazda Mazda5 205,000 mi,
Had same issue and also had to replace theramstat.
2006 Mazda Mazda5 71,000 mi,
Check Engine Light on - Code P0126
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!
2006 Mazda Mazda5 75,400 mi,
Check Engine light came on- code 126 a guy at pep boys recommended having the coolant thermostat replaced- payed $200 for parts + labor. the check engine light is off now so I guess problem solved?
Light came out stating that I had a low tire I have had all tired. Hecked and air added even the spare light is still on
2007 Mazda Mazda5 68,000 mi,
Check Engine Light on. Thermostat stuck open, replacement costs $282. Mostly labor.
2006 Mazda Mazda5 200,000 mi,
problem appeared at 100000 miles, I've replaced the thermostat, but problem reappeared within few weeks, I live with it since that, sometime the light goes off
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 !!!!!
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.
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.
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.
2007 Mazda Mazda5 68,000 mi,
Thermostat Replaced and Flushed Coolant $400 mostly labor and removal/new coolant
2006 Mazda Mazda5 91,000 mi,
CEL on Po126 error code on OBDII scanner, engine temp gauge appears to be in normal range. Also having AC cooling issues, not sure if linked.
engine light on due to stuck open thermostat
2007 Mazda Mazda5 57,936 mi,
Check engine light on and thermostat needed to be replaced