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Q: Engine light is on

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The engine light on our 2007 Mazda 3 came on. We purchased it thru a car dealer over a month ago. The code that appeared when the mechanic looked at it was/is P0126. The sensor was changed and the light stayed off for 2 days, but then came back on.

We took it back, being that it failed inspection and they drained and replaced the coolant. From there, the light stayed off for about a week and within that week, the car passed inspection.

The light is now back on, but the temp in the car has not changed and has been consistent since we purchased it and it is not running rough either.

Could the thermostat need to be changed? Again, a new sensor was put in after the coolant was replaced.

Any information that you can provide will be greatly appreciated!

My car has 68000 miles.
My car has an automatic transmission.

A: Hello. Diagnostic Trouble Codes (DTC’s) do ...

Hello. Diagnostic Trouble Codes (DTC’s) do not necessarily identify faulty parts. For example, the Code you are inquiring about, namely, P0126 could set the Check Engine Light (CEL) simply because the coolant level in your car is too low. If the liquid level is too low, the sensor will not work properly. No faulty part is involved in that scenario. To condemn the sensor, the mechanic would have had to use a scan tool to read out the temperature that the sensor is reporting to the PCM (engine computer) and compare that reported temperature against a physical temperature measurement at the sensor base. If the two match, there is nothing wrong with the sensor.

So, there might not have even been anything wrong with the sensor unless that diagnostic was done. Of course, the foregoing is relevant because you are reporting that after sensor was replaced, the CEL came back on. A key point is when you take your car for repair and they tell you such and so code(s) were set, so we are going to replace "Part X", you should ask, "Did you actually test and diagnose that specific part?" "What other possible causes are associated with the DTC and did you consider those other possible causes before condemning Part X?" DTC’s only give clues or indications as to where to look, but then a Mechanic has to separately diagnose or test each possible cause.

So, the sensor should have been tested and it probably wasn’t. Draining coolant and refilling is not a recognized repair strategy to resolve Code P0126. If you paid money for that, you are due a refund excepting if you agreed to it as preventive maintenance. If the present Code is still P0126, what a Mechanic (from YourMechanic for instance) would do is read the engine operating temperature as reported by the sensor using a scan tool and then simply see if that is the actual (physically) measured temperature. If the sensor is working (presumably it is as it is new but one still has to check the wiring to the sensor.

It could be the PCM is getting a faulty signal due to a wiring problem but this would NOT apply if the ACTUAL measured temperature matches the scan tool reading), reports an out of range temperature AND the physically measured engine operating temperature is in fact too low, you probably have a faulty thermostat. But, even before the thermostat is condemned you still have to check and make sure that the engine cooling fans are not stuck on, thus running continuously, and overcooling the engine. Also, the coolant level has to be re-checked just in case you have a leak. A certified technician from YourMechanic would be happy to walk you through all of this and, in the end, give you a Check Engine Light diagnosis that will permanently resolve this issue for you. Thanks for your question and best wishes to you on this.

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