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My CEL comes on every once in awhile then goes off the next day. Three weeks later it repeats the pattern. Yesterday I plugged in my code reader and got a P1117 oxygen sensor heating read out in the error code. I've gone on-line and see Bosch makes two sensors for my 01 C4 Cab, a # 15250 downstream and a # 13564 upstream. I don't want to order the wrong sensor. Anyone out there know the correct one ?

1117 Heating of Oxygen Sensor bank 1 (driver's side) behind catalytic converter, Possible Short to Ground or Below Lower Limit / Lean Mixture Limit

Some controversy whether whether before or after cat. I would go with AFTER
https://www.pca.org/tech/code-p1117-0