Stuck Thermostat?

Joined
May 19, 2018
Messages
84
Location
CT
Hi All, My '07 Accord 4cyl threw a p0128 code last week indicating "Coolant Temperature Below Thermostat Regulating Temperature." I hooked up my scan gauge and and on a 20f day, with about a 2 minute idle before leaving the house, the coolant temp was up to about 175f within about 5 minutes or two miles of driving and the car was throwing decent heat. Temps for the most part stayed between 170 and 180 for the rest of the commute with the exception of when I rolled down a 1/2 mile hill in top gear (5mt) and the temp dropped to about 165f. I believe the car has a 170f thermostat from the factory. Ran a test last night after letting the car cool for about 2 1/2 hours by starting it and letting it idle under no load with the heat off. Ambient temp was about 15f, water temp was about 75f. Within 10 minutes the water temp was up to 175 and by 15 minutes the water temp was up to 185. I then turned the heat on full. After about 5 minutes the water temp was back down to 147f. Does this sound like a thermostat is stuck open, or is this somewhat normal for a small engine in cold temps being asked to put out heat without much load? I noticed last winter that the temp gauge would start to drop when rolling down long hills, but it always kept throwing heat and never threw a code so I didn't think too much of it. Also, the main reason I am asking before just changing it out is because this is the first code I have even encountered that I cannot clear with my code reader. It says command sent, but codes remain. I don't want to change it out if this is more indicative of a bad sensor or something else. Any help/input appreciated.
 
Joined
Jan 9, 2010
Messages
12,047
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Los Gatos, CA
I just changed the thermostat in a 2004 Sentra with the same code. Then I cleared the codes with my cheapo OBD 2 module and cell phone. $12 fix; all good.
 
Joined
Dec 23, 2006
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Canuck - moved to —> California —> Texas —> ???
Yes, I had the same code with my Mazda 3. From the heater output and the dummy guage, I was not able to tell if the thermostat was bad, but the ECU measures that stuff, unlike us humans. Change the thermostat and the code should reset by itself after few cycles.
 
Joined
Dec 19, 2013
Messages
6,856
Location
Fort Worth, Texas
Originally Posted by JamesBond
Did it only happen once? I would reset the light and see if it comes back.
P0128 isn't usually a "One Trip" DTC, The PCM has noticed the engine either takes to long to reach operating temperature or it never reaches operating temperature MORE than once.
 
Joined
Jan 1, 2018
Messages
1,195
Location
San Antonio, TX
If the thermostat is stuck open, your engine will run cooler on the freeway; and it'll run cooler than the thermostat's temp rating. My water temp bounces back between 194 and 196 on the freeway. Before when it was stuck open, the water temp would stay at about 176 on the freeway. Fahrenheit.
 
Joined
Jan 21, 2011
Messages
2,283
Location
California
I'd like to know what temp thermostat is in there, compared to the temp readings you are getting. In my experience, if a Tstat is stuck open, it will take a very long time to get anywhere near operating temperature. 5 minutes is pretty good to reach 175F.
 
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Joined
Nov 30, 2018
Messages
84
Location
Somerset, UK
My V70 was similar when I bought it - it would warm up fast but never sit at full operating temperature. Changed the thermostat and now it's spot on every time.
 
Joined
Nov 20, 2006
Messages
28,651
Location
MA, Mittelfranken.de
The wax pellet is probably done for. Thermostats are not just an open and closed deal they constantly regulate by slowly opening and closing the gap as needed at operating temp, this happens much more frequently in cold conditions.
 
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