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 Mazda3 Check Engine Light/Trouble Code Due to Stuck Open Thermostat
Mazda Mazda3 Problem
Average Mileage: 87,918 mi (50,000 mi - 165,000 mi)
Visitor, 2008 Mazda Mazda3
Had to have the thermostat replaced 3 times in the past year and a half average bill for this is $500 nice to know Mazda is doing nothing about this problem
Visitor, 2007 Mazda Mazda3, 63,200 mi
Check engine light came on during recent cold weather. Took car to Auto Zone to have error code read. Error code returned P0126 and P0128. Checked Internet forums and determine faulty thermostat was most likely the issue. Purchased new thermostat at Mazda dealership for $65.
Replaced part myself. Check engine light now off.
Visitor, 2008 Mazda Mazda3, 80,000 mi
Had a check engine light and mechanic said it was a stuck thermostat.
Visitor, 2005 Mazda Mazda3, 107,345 mi
I just bought this vehicle and the engine light came on the day after I bought it!!! Autozone checked it out for me and the code directs us to the Thermostat!!! I noticed the timing belt is older so I will change it as well, the tensioner "pulley" is making a noice so I will take a look at it and hopefully with some grease I can get rid of the noice. I also notice a ratle on the driver's side front end which i'm guessing will be the shocks or maybe tie rod ends?
Visitor, 2004 Mazda Mazda3, 92,000 mi
Just bought car used and 3 days later check engine light came on. Took to Advanced Auto for code details and was diagnosed as thermostat being stuck open so.... here I am at day 5. Planning to take car to be fixed which dealer will not stand behind. $200 later.
Visitor, 2004 Mazda Mazda3, 130,000 mi
We just keep clearing the code and it stays off till the next long trip I take. It throws code P0126.
Visitor, 2004 Mazda Mazda3, 106,000 mi
CEL code P0126
Visitor, 2007 Mazda Mazda3, 50,000 mi
Check engine light came on with 3 codes and it was reset by the dealer. Advised to come back when the engine light comes on again which it did 4 months later. Dealer determined thermostat was stuck open and needed replacement.
Haven't had it replaced yet as I don't want to spend the $250+ to fix it. The mechanic said it has no bearing on the operation of the car and nothing is at risk; I would just need to pay for this before my car's next inspection.
Visitor, 2008 Mazda Mazda3, 84,000 mi
CEL has come on twice within the past year; both times code has related to thermostat stuck open. It went off on its own after about 2 weeks the last time, but has since come back on. I just called to get estimates...Dealership quotes $350 for parts & labor (!!), I have found a local mechanic who will do it for $165 labor and I can bring my own part, found the thermostat at Autozone online for $30. This is the route I will be going, and just hope it fixes the problem.