: O2 sensor-bank 1 S1

Harry Yarnell
10-24-13, 09:42 AM
'99 DeVille.
I've got a code 131 showing; rear O2 sensor. In an attempt to replace this sensor, I find the two coolant pipes DIRECTLY overtop the sensor. No tool I had or could get would get onto the sensor. Even if I used a 7/8" crowsfoot wrench. I couldn't raise the sensor past the pipes. Only solution was to move those pipes; I've done this on another Northstar (in the car) to replace rotten ones, and I swore I'd never do it again; VERY labor intensive.
So I cheated...
I dug into the white connector to the PCM, and cut the leads to the rear O2 sensor, and soldered them (the input to the PCM) to the leads to the FRONT O2 sensor. This way, the PCM THINKS it's looking at the rear sensor, but is really the front sensor feeding the PCM's two inputs. It's a crutch, I know, and maybe I'll address those pipes one day if and when I drop the motor for something, like, I don't know, headgaskets.
Car runs great...and no CE lamp.
I'm open to any suggestions or feedback; flame suit on...

10-24-13, 09:47 AM
Flame suit on.......lol

10-24-13, 10:26 AM
What tools where you using? Did you use an O2 sensor socket?


Harry Yarnell
10-24-13, 10:30 AM
Yes. I have two styles; the one you show, and a short stubby one that has an outboard 3/8 hole for an extension bar, sort of like a crowsfoot wrench

10-24-13, 10:39 AM
I've never had to do one Harry (and I'm NOT complaining :)) so I do not speak from experience, but maybe if you lower the front of the cradle a bit that would give you a little more clearance?

Harry Yarnell
10-24-13, 11:34 AM
You're right, lowering the front of the cradle will give me a better shot at the sensor. What I don't understand why those coolant pipes are right overtop the sensor; normally they're closer to the head. I suspect someone's been in there before.

Harry Yarnell
11-25-13, 05:19 PM
Well I bit the bullet and replaced the rear O2 sensor that was trapped by the two steel coolant pipes. Dropped the front of the cradle down for better access at the firewall, drained the coolant, undid the two rear clamps holding the pipes to the head, disconnected ONE coolant pipe on the drivers side, and maneuvered them just enough to get the tool on the sensor. Nothing broke and all fasteners cam loose without a fight. Now there are no codes and the car went thru Maryland emissions just fine.

Now, as I said origionally, I soldered the two inputs to the PCM together so the computer would THINK it was seeing both banks (the rear sensor was out of the circuit) when it actually was only seeing the front bank. This should have worked in theory, but would set code 171 (lean mixture, rear bank) after maybe 20 miles. How does it know? The only way is from the rear O2 sensor, but that's out of the circuit.

At any rate, the code 171 has disappeared as has the 131
Life is good.

11-26-13, 10:16 AM
U probably noticed when motor tilted forward, the pipes which are bolted to motor also tilted forward.

Harry Yarnell
11-26-13, 06:49 PM
Not sure what your point is. The reason to tilt the engine is to gain access to those pipes.