I35GEEK

I received a P0128 code while driving my car for about an hour when the outside temperature was 35 degrees F. Upon searching the forums, I realized that it is either a faulty engine coolant sensor or a thermostat stuck open.

I went ahead and replaced the engine coolant sensor since it was a cheap $10 fix and my old car has the original sensor. I replaced my thermostat less than a year ago when I did some timing chain work. It is not OEM. So it is possible that the new thermostat has gone bad quickly.

I started to use my OBD II reader and read the live coolant temperature data just to learn what is going on. When my car is moving mostly in 40-60 mile hour traffic, the car runs around 170 degrees F. Only when I am idling will the coolant temperature ever climb to over 190 degrees. Is this considered normal? It never reaches over 175 degrees when my car is moving.

My next project. I am going to take measurements with an infrared temperature gun to read temperature differences between upper and lower radiator hose. That will allow me to determine if I have a bad thermostat is my understanding.

I have never taken live readings of coolant temperatures before. But is it normal to be only in the 170’s degrees when the car is moving and only when the car is idling will it reach above 190 degrees. I live in the south so it was almost 60 degrees outside when I took these readings yesterday. Your thoughts are most appreciated.

CRBWS6

iTrader: (1)

In 5-10 minutes it should warm up to 177-182 deg (FSM p. LC-18 valve opening temp range) or a little higher.

You are correct, about the temp delta between the upper (hot!) and lower (warm but less hot than upper). You can check this with your hands - the difference will be pretty noticeable. If there is not much (or a huge) delta, the t-stat is probably bad.

I suggest an OEM t-stat. I have never had an issue with OEM (Nissan, GM, etc) but have had issues with aftermarket (Napa specifically).

I have found that my OBD reader data matches the OEM tstat spec pretty close in cooler weather (a little higher than spec in summer). E.g. my truck has a 187 deg t-stat and last time I data logged, it was reading 190-194 deg on a 20 min freeway drive after about 7 min to warm up (55 deg day).

I35GEEK

Thanks @CRBWS6 for your input. After measuring the ECT using OBD 2 and the upper & lower hoses using an infrared red thermometer, I was able to achieve a graph similar to this from https://www.samarins.com/glossary/thermostat.html


So I am assuming that my thermostat is working. The check engine light has not returned yet after the ECT sensor replacement. So hopefully, that is the end of this car drama.

Prophecy99

iTrader: (1)

yeah I got this code once and a ECTS replacement fixed it. thank you Dennis.

https://maxima.org/forums/5th-genera...same-time.html

I35GEEK

I continued to do a few more readings of my ECT and hoses. Here is what I learned. When my car is moving, the ECT is somewhere in the 170’s degrees. When I am idling at the traffic light, it will climb to 180’s. This is in line with when the thermostat opens.

I parked my car in my driveway and watched the temperature slowly climbed to 190’s and then at 203 degrees, the radiator fan kicks in. It keeps it under 203.

I now learn that ECT can fluctuate from 170’s to 200’s depending on whether the car is moving or idling. Good to know.