Car Problem Reports

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

Mazda Mazda5 Problem
24 Reports
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Model Years Affected: 2006, 2007, 2008

Average Mileage: 71,917 mi (26,383 mi - 101,000 mi)


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).

  • Visitor, , 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.

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  • Visitor, , 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.

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  • Visitor, , 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!

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  • Visitor, , 71,000 mi

    Check Engine Light on - Code P0126

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  • Visitor, , 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.

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  • Visitor, , 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.

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  • Visitor, , 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.

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  • Visitor, , 68,000 mi

    Thermostat Replaced and Flushed Coolant $400 mostly labor and removal/new coolant

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  • Visitor, , 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.

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  • engine light on due to stuck open thermostat

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