My Rheem furnace lights up runs about 20 seconds burner goes out the n relights for another 20 seconds then goes out

Sounds like a dirty flame senser. Remove it, clean it with steel wool, or find sand paper, and put it back in. Should take care of the problem. Most Rheems have three diagnostic lights. Two green, and one orange. The two green should always be on steady. The orange should come on steady once the flame is ignited. If the orange fails to come on, or flashes, the flame senser is dirty.