P0128 code on 04 grand marquis

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Recently picked up a great grand marquis with handling package. CEL came on and code reads P0128. Says Coolant Temp Below Thermostat Regulating Temperature.
Anyone else ever see this? Drove car to Florida and back and drove great. Fuel economy seemed a little lower than I hoped but not terrible. Could get 25-26 at 70 mph dropped fast going faster though.
 
The most common cause for P0128 in a Ford is the engine coolant thermostat is stuck open. ... When the thermostat does open, hot coolant should start to flow and quickly warm up the radiator hose. If the radiator hose heats up slowly, the thermostat is stuck open or opening prematurely and needs to be replaced.Oct 11, 2018
 
Recently picked up a great grand marquis with handling package. CEL came on and code reads P0128. Says Coolant Temp Below Thermostat Regulating Temperature.
Anyone else ever see this? Drove car to Florida and back and drove great. Fuel economy seemed a little lower than I hoped but not terrible. Could get 25-26 at 70 mph dropped fast going faster though.

Most likely the thermostat was removed by a previous owner for whatever reason.

As long as the CEL remains illuminated, the MPGs will be less than they could be if it were not illuminated.
 
I would suspect that a part of the intake manifold has broken off and is holding the thermostat open from my personal experience.

A failsafe thermostat may have also used and is hanging itself open, or there is no thermostat at all.
 
Agree, probably stuck thermostat should be replaced, then ideally, cooling system flush and fill. The code may not go away till you do a full drive cycle resetting the ECM, or clear code with a scan tool.
 
Use a scanner to read the coolant temperature after the car has been parked long enough to fully cool off, it should be near the outside temperature. Then start up and test drive to see if it heats up as expected. Coolant temperature sensors can go out of spec and send wrong readings, but not so far wrong that a sensor code is set.

Being a newly acquired car, as others said you almost have to physically inspect the thermostat. Parts stores sell thermostats that are lower than stock temperature-- do not use those.
 
On older cars, it is common for some people to remove the thermostat when they have cooling problems… So when you've installed the new thermostat and summer comes around, expect to have some overheating problems. I would suspect that the radiator is partially clogged or has scale, reducing its cooling ability.
 
P0128 as stated above - the car is not seeing the expected temp (it is lower than should be) so it sets this code. Also, as stated above - the #1 reason is a stuck open or sloe to open thermostat. I've seen this code 100's of times and a new thermostat fixed it each time.
 
I had this exact problem on a Fiat. Usually it's a mechanical issue with the thermostat. I replaced mine and problem solved.
 
Drove car to Florida no overheating issues in 75-80 degree weather. When it warms up a bit plan to crack it open and inspect/replace thermostat. Radiator and coolant look new but car was in light front end accident so maybe a new radiator. Definitely got new headlights. Airbags were not deployed front or side.
 
Interestingly the check engine light went off on it's own yesterday probably the coldest day of the year that I drove it. You would think if it's not getting up to temperature it would be worse on a cold day. Currently don't have a way to check actual engine temperature to verify sensor. All I can say is gauge looks low but not an actual reading just digital display. Wish they had a reading for computer temperature. My old 88 Toronado had a digital display that would tell me engine and transmission temperature.
 
Replace it. T-stat in my 357k mi 1uz had a long fruitful life. $6.39 later it's much better
 

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