Nissan Repair: 91 Nissan Sentra check engine light, nissan sentra, upper radiator hose

Question
I replaced the engine in my 91 Nissan Sentra, 1.6 L  2 weeks ago.  My first trip of over 10 km (went about 20 km), the check engine light came on.  I noticed that the temp gauge was 1/10th of the way to "hot".  When I first drove the car after putting in the new engine, the temp gauge (on a 4 km trip) went to 1/4 of the way to "hot".  I have a couple of questions:  first:  I felt the thermostat housing (on the passenger side of engine, facing towards back of car) and it was cool, while the hose leading into the water pump (from upper radiator hose) was hot.  Does this mean the thermostat was NOT opening?    Second:  The car is cool now, the check engine light stays on.  How do I access the computer in this model car?

Answer
Hi Bruce -

The thermostat is designed to open between 165 - 180 degrees. If the car has been running for a while, you are correct, you should feel it hot on both sides. If not, replace the thermostat. I would run it until the cooling fan goes on and you feel it hot on both sides. If the radiator bubbles and you hear knocking sounds (water boiling inside the radiator), then turn it off immediately, let it cool down and change the stat.

To access the computer codes, you will need a diagnostic tester. To ensure it is not an nuisance signal, disconnect the battery fomr the vehicle and wait 60 seconds, then replace it. This should reboot the computer. If the light still goes on, then you will have to get it diagnosed. Take care of the thermostst first though.

Hope this helps!
aloha
calvin