I'm trying to install a bosch universal 3 wire 15726 downstream o2 on my gf's 97 legacy 2.5 but the org. Was a 4 wire? Anyone else run into this? It's a bosh 15726. Thanks!

I figured it out.Bosch 3 wire is signal(black), heater+(white)and heater-(white) . 4 wire is all those plus sensor ground(grey).

Anyone have any experience with switching the 4 wire for a 3 on the downstream?

If the downstream has 4 wires then the replacement has to be a 4 wire sensor. No easy way around it. Take it back and exchange it for the correct sensor.

 

Some of these have a 4 wire plug for the rear sensor, but only three wires in the plug. Check the sensor harness carefully because there may be 4 wires in the extension harness that runs alongside the trans, but only 3 wires on the actual sensor.

Edited April 5, 201510 yr by Fairtax4me

Original sensor has 4 wires, most aftermarket are 3 , missing that grey ground. Cel is off, grey wire not connected.