2003 XC90 Check Engine and ECT problem

[susan_us2002]

Hi all,

This is my first post in this famous forum, even I got lots of useful info in the past several years from here.

3 years ago, got check engine light, dealer said it was thermostat problem. I bought OEM thermostat housing including coolant temp sensor. After that, I just gained 2000 miles on this car.

At the beginning of 2014, tranmission was broken, dealer installed new transmission and radiation etc. I drove several hundreds miles until now.

Last month, check engine light is on again, and dealer said ECT problem, and need to replace thermostat and coolant temp sensor. Sometimes, I see the temp gauge change to "0" from 1/2 suddently, and come back to 1/2 again after several mins.

My concerns are
1) Do I need to change both thermostat and coolant temp sensor? or just coolant temp sensor?
2) I know thermostat replacement is not easy, need to drop coolant, remove coolant tank and black cover ... How about sensor replacement, is the same as thermostat replacement? If it is easy, need to remove coolant tank etc?

Thanks for your reading.

Susan