Exhaust sensor

[BR3W CITY]

20" is a ballpark, but it works. Usually the idea is to have it a far "up" as you can, while still being in the collector so ALL the exhaust can mix before/while passing the sensor. This is for "normal" manifolds that bolt to a "mid" pipe, or on full length headers at the collector.

Mounting it too close to the exhaust ports (like on a short header setup), will put an extra amount of heat load (direct and indirect) on the sensors. The sensor can burn out faster and is more prone to erratic/false readings, and I wouldn't be surprised if the difference in exhaust pulse very close to the heads had an effect as well.
Too far away and you have the exhaust cooling down, as well as an increased likely hood that a clean air leak can occur before the sensor throwing off readings.