Seems like an odd combination of signals/codes simultaneously. "Check Emission System" light came on yesterday, so I ran OBDII scan and the only code was P0128, which is "Coolant Temperature Below Thermostat Regulating Temperature," and coolant temp remains below regulating temp even after driving for awhile.

Initial research is that the thermostat is likely stuck or broken in the open position, so not overheating. But the Emission System light, too? One source online says P0128 can trip other things, and another says to check air intake and coolant temp sensors. Since thermostat is mechanical and not electronic, it shouldn't be affected by a bad sensor should it?

Thanks, all!

eta: My shop has an opening to drop by for a look, but I'd like to be armed with any thoughts from here if possible.

 

Has your temp guage been lower than normal after driving it for a while?

 

Yes, but gets close to but not quite normal when idling. My local shop said it's the thermostat. He wants $400+ for diag and OEM replacement t-stat + new coolant. I told him I want OEM Acura t-stat, which is $140 for Acura vs. $25 for aftermarket part.

Haven't replaced a coolant thermostat myself in decades, and it was accessible and easy on my '68 Camaro V-8. Is this accessible and easy, or a project?

Thanks.

 

The job is very simple, the book gives you an hour and a half to replace it.

 

Thanks, Altair47. Haven't tackled anything in the cooling system myself on this car. Looks fairly straightforward.

 

Update: Ordered OEM thermostat online and plan to do job this week. But, it bothered me that the check emission light came on almost immediately upon starting the engine before time to warm up, leading me to believe that there's an electrical issue e.g. a bad sensor besides just the mechanical thermostat. Suspecting that maybe the engine coolant sensor is bad, too, so I ordered one to swap out at same time as thermostat after draining the coolant.

Then, I learned that there are not just one but two coolant sensors; one on the engine block near the thermostat, and the other at the radiator near the drain! Some online parts stores only show one diagram or the other, leaving me wondering at first whether there's only one sensor but in different places for different years, but then one supplier showed both diagrams for my '13 RDX and sensors in both locations. Apparently same sensor for both locations, #37870-RTA-005.

I suppose I should order a second coolant temperature sensor and replace both and be done with it. Am I on the right track, or off the rails?

 

What's stopping you from starting a cold engine and, as it warms up, touching the heater core the hoses and the hoses on the radiator?

 

OK, did that. They eventually heated up to normal operating temp, but took nearly 15 minutes of idling in garage with heater off to get to normal operating temp. Coolant is circulating fine, and temp gauge rises but very slowly. So, I take it that the termostat is stuck in the "open" position. The "Check Emission System" light and chime came on just a few seconds after starting the engine, even after clearing it, and of course stays on. That symptom arose at the same time as the stuck open thermostat.

Edit to add: Actually, I started it with the heater on, but turned it off after several minutes to get engine to come up to temp.

 

Simple and quick diagnostics, and you don't need any temp sensors.
It's amazing that the original thermostat lasted almost 250k miles.

 

OK, looks like I can skip the sensors. Thanks very much for your help!