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: 70,907 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, 71,000 mi
Check Engine Light on - Code P0126
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.
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, 2007 Mazda Mazda5, 68,000 mi
Thermostat Replaced and Flushed Coolant $400 mostly labor and removal/new coolant
Visitor, 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.
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
engine light on due to stuck open thermostat
Visitor, 2007 Mazda Mazda5, 57,936 mi
Check engine light on and thermostat needed to be replaced
Visitor, 2008 Mazda Mazda5, 63,000 mi
Car overheating. Check engine light on. Car doesnt overheat if interior vents are open at full blast. Called Mazda to find out if they plan on fixing this situation and they said "we cant help you car is out of warrenty". Too many people are having this same problem this is a manufacture problem please call Mazda and report it so they are aware. Mazda # 1800 222 5500. Even if you fixed problem call please.