i changed the oil pan on my friends 2000 accord v6 and to do so you remove the twin down pipes which has the upstream o2 sensor and connects right in front of the cat. i unplugged the sensor and finished the job double checking all my connections. but when it was started it has the the codes p0131 low voltage and p0132 high voltage even tho there was no codes before and i didnt touch the sensor except the un plug it. any thoughts

 

Oil/dirt or some kind of gasket-goop?? The sensor is very easily damaged by silanes (RTV silicone and some exhaust goop products). Leave it go for a couple days, then reset ECU & see if the code comes back.