i was driving for like 30 miuntes, my cel came on and i tested it when i got home... got code 1, 21, 41... can someone help me figure this out? like why would i get 21? also my o2 sensor wasnt reading any voltage when my uncle checked it one day, i never looked into it. but that was awhile ago, and idk why the cel would just come on now.. and ideas are helpful. and any way to test the sensor?