I really need some help from this group please. I have a 2004 Lariat that keeps throwing a P0128 code "engine coolant below regulating threshold". I have replaced the thermostat twice with 190 degree thermostat from the dealership and that didn't fix it. Then I replaced the engine coolant temperature (ECT) sensor and that didn't fix it. So then I replaced the fan and it is still throwing the code. When I am on the highway the temp gauge goes up to about a quarter of the way. In town the gauge operates normal right in the middle of the temp gauge. Get back on the highway temp gauge goes back down to a quarter reading. Has anyone else ever experienced this? I am at my wits end and would really appreciate any recommendations. Thanks in advance.

 

Are you talking about the CHT sensor? Or did you not do the work yourself and take it to a shop?

 

Didn't know we had a ECT sensor.. what year truck and motor/mileage do you have Mark ?

 

Agreed @techrep - would ALWAYS be more able to help if we have full vehicle info. IF his truck is a Ranger - ------ I just ran across the following text in the Motorcraft OBDII Theory of Operation Manual. I almost BET it explains his problem!


"Some vehicle applications (2.3L Ranger) use an electric thermostat heater. These vehicles use a high temperature thermostat (220 oF versus 192 oF) to achieve faster warm-up times. The heater circuit can be energized by the PCM whenever additional cooling is required. (The PCM energizes the heater based primarily on ECT/CHT, but can allow for additional cooling based on inputs from rpm, load, IAT and TFT.) The heat generated by the heater causes the thermostat to open at a lower temperature than the rated temperature of the thermostat (up to 50 or lower), thereby, providing additional engine and transmission cooling. The PCM duty cycles the heater output at 100% to open the open the thermostat, 70% to keep it open and 0% to provided rated thermostat function. The PCM monitors the "smart" driver fault status bit that indicates either an open circuit, short to power or short to ground. If the heater circuit fails such that it is always off, the vehicle can run hotter than normal. If the heater fails such that it is always on, the vehicle may also fail the thermostat test (P0125/P0128). Thermostat Heater "

 

Do they make a Lariat Ranger?

 

Well hell, you got a good point - I guess so or maybe they made a Lariat with thermostat heater the fails in such a way the vehicle fails the thermostat test (P0125/P0128).

 

We must.......My 07 5.4 does because on my Gryphon I monitor ECT and they max at about 196-198 fully warmed up.

 

Sounds like a clogged up radiator.......When was the last full flush done on the cooling system? Do you off-road a lot in the mud?

 

Clogged radiator would be running hot. I think the OP is running to cool. I got the same message on my 06 4.6 using my Ultra Gauge recently. I was up north for Tgiving and headed home. Outside temperature mid 20s and about 3-5 minutes in to my trip home got the MSG "engine coolant below regulating threshold" and checked my temp and it was 145. After a few miles warmed up to my normal temp which is always in the 180-186 range and drove the 500 miles home. Didn't get a CEL just an Alarm MSG on my Ultra Gauge. I figured it was due to the cold weather and taking longer to warm up the engine.

 

Are you running in sub zero conditions -canada or something , Ambient please?
What engine -year . What state --elevation?
Do you have an extra oil cooler . Is your transmission atf up to full .
Are you changing thermo to pcm or gauge . Is the engine physically cold to touch . Borrow an infrared temp gun and point it at engine .
How long of trip have you done ,does it change after running a long while . Are you getting black smoke out of exhaust? PCM may run it rich if it thinks its cold .Is it really cold or wiring problem ,rats etc.