2000 Ford Ranger 4.0L running too cold?

Got a 2000 Ford Ranger 4x2 super-cab with a 4.0L and automatic transmission that is running too cold (about 140 deg f). There is a code that says "ECT temp too cold for closed-loop fuel control". I did a tune-up on it and changed the thermostat (thought was sticking open). When you squeeze the upper radiator hose it is firm and doesn't compress. The upper radiator hose is hot and the bottom one is cold. The radiator cap is cold. The vehicle was towed behind an RV for the majority of it's life and just received it's first scheduled maintenance. The only thing I can think of is the radiator is plugged, can you help?

[98blownranger]

try to flush it out. there is a drain valve so you don't have to remove any hoses. being towed should not matter, the water pump doesn't move when being towed