Before you just go ahead and replace the thermostat you should do a google search on code P0128 to see what it's all about. It may not even be your thermostat. Maybe your temperature sending unit is dirty and giving a low reading for example.
How to replace it? You should get a Haynes manual to give you general guidelines and torque specs.
Basically you will;
Drain the cooling system down
Remove the alternator
remove the water inlet
remove the thermostat
replace the thermostat gasket
replace the thermostat - make sure the jiggle valve is on top
install the water inlet - don't overtighten- torque is very small - 8 ft-lb
Install the alternator - torque 12mm head/18 ft-lb, 14mm head/39 in-lb.
Check for leaks
Thanks for the reply, I am planning on picking up the Haynes manual today after work. Is there any chance that the thermostat can be removed without removing the alternator? I saw a thread on here where somebody just removed the thermostat bolts and voila, it was out without having to do everything else the book recommended.
P0128 just says thermostat malfunction and I haven't found a lot of information about it. It does seem like the thermostat is stuck open because the car takes a long time to warm up. Any other advice would be greatly appreciated. Thanks again.
If you search for P0128 in Google you get 22,600 hits. Lots of forums from other cars say that they have replaced the thermostat but the code comes back. Of course a lot of them also say that the code never came back. Just lookin' at all the options before assuming it's the thermostat. It is a good clue that it is the thermostat when you say the car takes a long time to warm up.
I quoted the brief instructions about removal from the Corolla repair manual - without actually looking under the hood. I'll do that tonight after I leave work. I agree that some operations described in the manuals (including Haynes) don't always have to be done in order to achieve the objective. It's certainly worth a look and if you can do it without removing the alternator - great!