P0126 - Thermostat or Sensor

I didn't risk it. AZ does OBD-II inspections now through the jack. So even if I cleared the code, I would be worried that the ECM will tell the inspection computer that the CEL has only been reset manually, not by repair/correction.

Here are the test conditions on AllData.

DTC P0126

-If the ECT signal never exceeds 160F after the engine starts for a specified period, PCM determines the coolant thermostat is stuck open.
Monitoring Conditions
-IAT above 14F
Vehicle Speed over 3.7 mph

Test period varies based on temp. For example:
If IAT is 14F, test period is 20 minutes.
If IAT is 86F, test period is only 8 minutes.

So it only runs the test if its warmer than 14 degrees, and it only runs the test if you are moving.

PS. I did the job. It really wasnt that bad, other than all the screws for the bottom cover were totally rusted and the clips broken, so I had to cut it off.

A couple of tips...

- Dont remove the radiator drain plug completely, the coolant shoots everywhere. Just unscrew it until you see some fluid start dripping straight down.
- when you remove any of the hoses, expect coolant to come out. Also, when you take off the thermostat, another large volume of coolant is going to come out. Be prepared with your pan.

- Definitely remove the tensioner. Its really easy, and it gives you precious inches for wrench swing. Make sure you pay close attention to how the belt sits on the on the pulleys before you remove it

If I didnt have to battle with my bottom plastic cover, this would have been a 1 hour job, 2 if you include burping the air bubbles out of the system after you refill the coolant.

Thanks for all your support guys!

rideburton56 said:

Here are the test conditions on AllData.

DTC P0126

-If the ECT signal never exceeds 160F (71.1C) after the engine starts for a specified period, PCM determines the coolant thermostat is stuck open.
Monitoring Conditions
-IAT above 14F (-10C)
Vehicle Speed over 3.7 mph (6.0kph)

Test period varies based on temp. For example:
If IAT is 14F (-10C), test period is 20 minutes.
If IAT is 86F (+30C), test period is only 8 minutes.

Click to expand...

Hey this is really interesting! I added metric equivalents in bold.

I have a 2006 Mazda 5 with 130k km (80k miles) and I had suspected my thermostat was leaking all this bitterly cold winter we have been having in Southern Ontario Canada. I suspect it but never saw the code until today. The dashboard guage always showed in the middle, but these guages usually do in most cars until something is way off. I would not be surprised if there is a separate sensor circuit for the dash vs the computer as I know there is in my Volkswagen. The sensor for the dash in the Volkswagen has a wide range of "normal" where it will read in the center even if the engine is not up to temperature.

Anyway, in the Mazda, I always had heat from the vents, but not as much as or as quickly as I would like. I have a scangauge and I know normal operating temperature is between 85C and 90C (185F to 194F) but this winter, it has been running between 70C and 80C (158F to 176F) even when the dashboard is saying "everything is normal". This, however, is not normal and has been stated, running an engine below temp is not good for it. It would only get down to the low end on extremely cold days when coasting in gear going down hill so I knew I had a minor leak. Anyway, today should be the last really cold day this winter so I will change the thermostat some time this summer probably.

To the person who wanted to lower their transmission temperature by lowering the coolant temperature, this is very bad idea for the reasons of wear and lower fuel economy stated above!

The engine coolant temp info is shared between instrument cluster and PCM over the data network on the 5 and most modern Mazdas. Only one temp sensor required. The only reason a manufacturer would still put two in is as a backup in case the main sensor fails. For emissions reasons the engine needs to warm up as quickly as possible. If it's not then you get p0126 and the check engine light. The difference is not enough to notice using the cluster gauge. It's all about how long it takes to reach operating temp from cold start.