Service engine soon light

I have a Chevy S10 (1993) truck. When I start the truck in the morning it is perfect. But after driving 10-15 minutes the "service engine soon" light comes on and the vehicle starts to stall. I took the vehicle several times to the local mechanic, but it seems that they cannot understand the problem. First time they replaced the distribution cap and rotor button, then they changed the engine coolant, temperature sensor and temperature sending switch (they thought vehicle control module was not getting correct engine temp). After all these work DTC 15 was found by another workshop (as the first workshop did not have proper scanner to read the error code) and suggested to check the circuit for sensor. When I told this to the previous workshop they told me that they already worked on the sensor circuit.