I know this issue may have already been addressed. My 2006 Miata MX-5 keeps getting that pesky check engine light, P0126 error code. I hooked up my scanner and started my cold engine and I monitored my engine coolant temp. After warming the engine thoroughly my scanner showed a coolant temp of approximately 125 degrees and it stayed around that temp for about 8-9 minutes and only then did it increase to around 205 degrees. Is this a sign of a sticky thermostat? Is there a thread addressing this specific situation?

 

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Your DTC is reporting there are no set rules or what we call anomalous DTC so the technician must follow a path of checking many different cooling system operations. Reading from the ECU how the coolant temperature is doing does not always point to a thermostat. You would need to use a hand help thermal meter and test the coolant temperature at the upper and lower hoses on the radiator and also the thermostat housing. Make sure that the radiator fans are also operating correctly. You can either check the operation using a Scan tool that can register the fan control or by doing a quick test by turning on the AC to max.
If the fans are working pull the cooling fan relay so your testing does not allow them to activate while you are testing. It may in the end be as simple as the coolant temperature sensor. You can also test that but require is be removed. And is after you do the other tests first.