Yes, most likely the thermostat is stuck opened. This code basically means that the engine is not warming up adequately as reported to the ECU via the engine coolant temperature (ECT) sensor.
If you hook up a scanner tool and read out the coolant temperature you will likely see the problem. Basically the engine temperature will be constantly hovering around 149F-158F. In that condition the ECU cannot go into close loop mode and it will also be enriching the fuel above normal operating conditions. That's why you're seeing poor gas mileage.
Wintertime conditions will worsen this even further because the ambient temperature is already quite low to start with, and a vehicle in motion will cause the coolant flowing through the radiator to cool down further, and finally since we normally operate the heater this will drive the coolant temperature down even further.
So I went to change out the thermostat today only to find that there wasn't even one in the car...I called the previous owner and he apparently had it removed when he had the coolant flushed a few years back with the idea that Florida cars don't need a thermostat. Misinformed people make me laugh. The guy never saw the true gas mileage this car could get. The cold weather recently must of triggered the check engine light. Glad I took care of this. Turned code off with scanner, lets see if it comes back on.
Unfortunately I think I may have to do this again. I am not sure I let the rtv silicone goop dry long enough as there is a small leak from the thermostat housing now. My question is should i use the paper gasket that was provided or stick with the rtv? I figured i would try to stick with the rtv first because thats what was there and it was working...but how do them duralast paper gaskets hold up?
What if I RTV around the T stat where the edge sits in the housing, then use the paper gasket for the housing? And if not, how long should I let the car sit before I refill the system? I am realling hoping that within the next 24 hours the RTV cures completely and stops the leak, but I doubt that will happen so how long should I let it sit this time to avoid this again?
Edit: Also, what about some rtv on both sides of the gasket that comes with the T stat? I really want to make sure I don't have to do this again as I rarely have time.
The AutoGuide.com network consists of the largest network of enthusiast-owned enthusiast-operated automotive communities.
AutoGuide.com provides the latest car reviews, auto show coverage, new car prices, and automotive news. The AutoGuide network operates more than 100 automotive forums where our users consult peers for shopping information and advice, and share opinions as a community.