Rep Power: 0

Have a 2002 civic 1.8L that is setting a p0128 code. The analog temp gauge takes about 25 min of highway driving to finally come up abit. Replaced thermostat and the check engine light came back on, and the gauge was very slow to move off cold. Any ideas? I was thinking it was a bad CHT sensor but that doesnt explain why the ECT sensor was reading cold too.

 
Rep Power: 517

Searched for "P0128", came back with this thread: https://www.civicforums.com/forums/182-7th-gen/349498-code-p0128.html

I'd revisit your thermostat. Use factory parts, install correctly.

 
Rep Power: 252

1.8 l?

 
Rep Power: 517

Good catch.
Not a N.A. market car?

 
Rep Power: 811

i dont think engine light will come on due to thermostat being bad. it will either take forever to warm up or overheat quickly. seems like a sensor issue. make sure you use honda oem thermostat and ECT sensor. not sure what else can trigger that code other than the coolant temp sensor (which also controls the gauge reading as well as vital engine performance).

 
Rep Power: 185

I'm pretty sure it will if the temperature doesn't come up in a specified time. I've seen myself on other vehicles. It just says basically low coolant temperature, which would lead me to believe it could possibly be a stuck open thermostat. And the thermostat I saw was indeed stuck open.