2011 Mercedez Benz E350 Base got p0128 error code on and off.

Question

2011 Mercedes Benz E350 Base got p0128 error code. Check Engine Light is on and off. The check engine light came on and stayed on a couple of weeks then turned off by itself.
Few weeks later the check engine light light came back. It was Yellow light not Red.
However, the car runs great and normal. Also the coolant temperature gauge shows right.
A month ago got dealer's recall service. They replaced the battery and installed a pin to somewhere to prevent faulty thingy.
The check engine light first came on few weeks after the dealer service. Not immediately.
The dealer ran the OBD2 and recommended the thermostat replacement.
Before do that I would like to check the all electric connections related to this error code p0128.
Please tell me where I should check?
Thanks a lot.