I'm working on an old 1994 Silverado with OBD1. I had an check engine light (code 21) that pointed to a bad TPS. Supposedly a good TPS should show less than .74 volts at idle (no pedal) and gradually rise to 5 volts at full throttle. I've now installed two different replacement parts that show good at idle and only 2.5 volts at full throttle. Am I measuring something wrong or missing something?