2000 civic, new header new trouble codes

thought i'd post this here see if anyone else has had the same problem

a friend put a new header on his engine. he took the old o2 sensor and put it in the new exhaust. swears he didn't cut the tail but he did untangle it from the wiring harness a little to get enough slack to wrap it round the new header.

i pulled his codes for him po131, po132, p420. i'm suspicious of codes that appear first thing after changing a part so i erased them and will see tomorrow if they come back.

thing i don't get is how can you get two codes that would seem to contradict one another. specificially

P0131 Primary Heated Oxygen Sensor (Primary HO2S) (Sensor 1) Circuit Low Voltage
P0132 Primary Heated Oxygen Sensor (Primary HO2S) (Sensor 1) Circuit High Voltage

is it possible the sensor is OK bu the wiring is off? i didn't have time to look at the sensor cuz we're at work but he said he was careful not to leave the wires touching the header

also got code P0420 Catalyst System Efficiency Below Threshold.. what does that mean? i didn't know there was even a sensor in the cat other than hO2 #2

another thing i don't get is he tells me the car isn't using more gas than usual. whereas when i had an ho2s problem with my ford escort it was eating gas. if his o2 is fried but the fuel economy is still normal does that mean something, or do hondas just default to lean mixture???

if code comes back on, check for open or short in the wiring. if all looks good, change out o2 sensor. a digital multimeter will be good your electrical troubleshooting.

as for the P0420, if it comes back on, your cat is gone. as for running lean when the cel is on for the primary o2 sensors, it runs in open loop mode. basically richer. even when running richer, hondas still do not eat as much gas as other cars do.