Codes keep showing up

Good morning I have a 2009 western star 4900 Ex series 60 Detroit. I was constantly having codes showing up basically all the sensors example oil temp coolant temp showing voltage high voltage low cam sensor messed up I changed that one and every other sensor including cam sensor and timing sensor the wiring harness for the left side sensors was changed 4 years ago the right side harness the feeds all the other sensors I was having trouble with I just changed 2 weeks ago I had the codes cleared from the ecm all good for 2 weeks this morning I get code 15 which is oil or coolant temp sensor voltage low and I get a code 21 tps sensor voltage high Btw I've changed the tps sensor at the pedal plus the wire harness that comes off the pedal why am I still having codes show up what else do I need to change thanks in advance.