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Earlier this year, I had my CEL come on on my '05 Mazda 3 due to P0128 (asked about here). I cleared the code, and it never came back. Six months later, I have the code P0126 come up and now I'm wondering if I should be replacing my thermostat or doing something else? And my follow up question will be - how do I actually do the replacement or other suggested task?

INFO: My engine temperature on my dashboard shows that it is below the range of temperatures when I start my car, and stays there until I start driving around 2 miles. It also doesn't get to the middle of the temperature range until I am at highway speeds.

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You should think of the temperature gauge on the dash as more of an indication of the engines temperature rather then the engines actual temperature. Some generic OEM ECU programme maps will replace a faulty temperature reading with a substitute value to operate the vehicle and in some/most cases you would not notice the difference. Use an OBD2 scanner to see the reading from your engine temperature sensor, OBD2 readings are the actual readings of the sensors and are never substituted, generic/OEM are occasionly substituted. Checking the engine with an infra red thermometer is also a good check to confirm or cross reference temperatures. Also with an OBD2 scanner check that all the monitors have run. You need to run a drive cycle for the vehicle if they are not all set to completed. If there is a secondry DTC present then a lot of earlier ECU drive cycle programmes will not run at all and you could drive around indefinitely with a fault which would not turn on the check light. A drive cycle must be run exactly, even having the radio on or off can effect the running of earlier set ups. Your thermostat may be electrically heated, check for code P0597/0598/0599. Check the sensors and monitors with OBD2 first.

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  • You clearly know more than I do about cars, so I want to ask some clarifying questions. What is an ECU program? I used my OBD2 scanner and the only codes coming up are P0126 and now P0128 (its back). The only thing my OBD2 Scanner (amzn.to/1hDH1FS) certain monitors are complete, nothing about temps. I don't have an IR Thermometer. What does it mean to run a drive cycle? How do I do that? What is a Secondary DTC? Thanks for the help so far!
    – tarheel
    Dec 7, 2013 at 6:09
  • In deciding how to give a credible answer to additional questions I asked a friend. The answer we came up with is to recommemd a book, "OBDII Functions, Monitors, and Diagnostic Techniques - by Al Santini" This book has been described to me as a little gem. The tool you have is not a scanner. It is a code reader. A code reader offers only the minimum of information ie the code. The code should also have a descriptor. A scanner will display information from OBDII modes, explained fully in the book.
    – Allan Osborne
    Dec 7, 2013 at 10:00
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If all else fails check to see your thermostat is opening and closing at correct temps. all you need is hot water and a thermometer.Workshop manual particular to your car explains how.Dead easy

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If you drop by your local plumbing shop and ask them for a "test thermometer" with a range up to 212*F, they'll sell you a neat little tool to keep in your toolbox. It'll have a fully round dial with a 6" steel probe sticking out of its back. The tip of the probe is the sensitive spot.

It's a fully mechanical device, never needs batteries, has no electronics to die, and may last past the end of your own life.

When it doubt about something like this, a test thermometer is the low-tech (reliable) way to determine temperature - insert the probe tip into a bolt hole on the intake manifold, or electrical-tape it to the upper radiator hose, or push it firmly (but not enough to damage things) between a pair of radiator fins near the upper radiator hose. It'll tell you what your temperature really IS.

Given your history, including the two OBDII codes and also given that your temp gauge is so slow to rise, and further given that a thermostat is a simple & cheap thing to replace... I'd suggest you swap it out with a known good thermostat and see what glints up from your goldpan.

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You could simply have a bad sensor. Or two bad sensors.

To test if your thermostat is functioning, start you car and let it idle while sitting motionless for about 5 minutes (longer on a cold day). Meanwhile turn on your heater and heater blower (fan). If the thermostat is bad (frozen in the closed position), it will not open after the engine warms up, and warm water will not be routed through the heater core, and you will not feel warm air blow on your face.

It may be possible for a thermostat to freeze in the open position, but I've never heard of it. In that case, it would take longer than normal for your heater to warm your face. Otherwise the car should function normally.

If your radiator fan is electric, it also should start up once the engine warms up. If not, probably that sensor is bad.

But I know of no way to test the sensors themselves other than to replace each sensor in question and see if the error code then goes away. Such sensors sit in water and often fail after about 10 years of use.

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