Temp gauge cause of 'service engine' light?

Hi

OK...heres the truck:
94 Full-Size Blazer 4x4
5.7L TBI
I've had a new crate engine installed last year along with a new rad, thermostat, & water pump. Recently, i also replaced my muffler with a Flowmaster model.

The problem I am having is that as I drive my truck and the engine warms up, the temperature gauge never reads beyond 50-60 degrees celcius. At the same time, after a short while of driving, especially at a constant high speed (110km/hr), the 'SERVICE ENGINE' light comes on. I have yet to notice any abnormal engine behavior, sounds or smells, and it seems after slowing down or stopping for a short time, the light goes off. However, the temperature never increases to the normal operating temp of 90-100 degrees. Coolant levels are fine.

I am curious as to whether or not the thermostat itself is at fault and staying open. If this is so, I must be suffering poorer fuel economy and hope I am not ruining the engine.

I have purchased a new TEMPERATURE SENDER (sensor?) but would like the expertise of those of you on here before I go and replace a $35 part for no reason.

Any help or suggestions would be greatly appreciated.

Cheers

Before wasting any time replacing parts, have the code pulled and lets find out what is going on.
Possibly could be something as simple as a stuck open thermostat.

i had the same problem with a 1993 olds cutlass, the service engine light came on and the car did not heat up, also very bad gas mileage and sometime wouldnt start or stall, we let a off duty GM tech look at it and he used a code reader on it, it came up to be the termorstat stuck open, flooding the engine with fuel because the computer was trying to warm up the motor, let me know what you find out, also thermostat only run about 5 bucks, jesse

There is a coolant temp sensor and a water temp switch on your truck. The coolant temp sensor is in the intake manifold and it tells the ecm what temp your truck is running at, the water temp switch is in the left head bank between your 2 and 4 plug holes and it is what makes the guage in your dash work. I have also found that using an infared heat gun all so helps in checking to see if they are working properly.