I just did a self test on my 88 f250 351w and got some codes that i dont know the meaning of. I got codes 21,24. ECT out of self test range .3-3.7 volts.ACT sensor out of self test range .3-3.7 volts. I dont know what this means and i was wondering if somebody could provide some insight on how to fix this.

 

You'll get the ECT code if you test when the truck isn't up to operating temperature. If the truck was cold when you ran the codes, don't worry about the ECT one. ACT is Air Charge Temperature, that's the sensor that senses the temperature of the air going into the engine. That one would be caused by a bad ACT sensor or wiring issue.

Erase the codes and then go drive it for about 10-15 minutes and dump the codes again. I doubt you'll get the ECT code, but if you get the ACT code again it's either time to replace the sensor or start checking wiring.

 

Agreed. More information here:

Fuel Injection Technical Library