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: 90,929 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, 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, 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, 2008 Mazda Mazda3, 80,000 mi
Had a check engine light and mechanic said it was a stuck thermostat.
Visitor, 2004 Mazda Mazda3, 106,000 mi
CEL code P0126
Visitor, 2007 Mazda Mazda3, 147,000 mi
engine light for stuck thermostat
Check engine light comes on. Mazda dealer says the thermostat needs replaced along with PCM update. $480.
Visitor, 2005 Mazda Mazda3, 113,000 mi
Engine light was on. Determined it was the thermostat stuck open. Thermostat had to be replaced.