Contour Enthusiasts Group Archives Forums General Troubleshooting Reversed O2 sensor codes?

I recently took my 1996 Contour SE (V6, mtx) to have a CEL diagnosis. The code showed a bad front O2 sensor, so the shop replaced it.

The light went back on within a few miles, so back to the shop it went. Same code on the brand new sensor. My mechanic did a little detective work, and believes he discovered a conflict between the wiring diagram and the computer. So he replaced the rear O2 sensor instead, even though the code was pointing to the front O2 sensor, and the problem went away.

So he says either the computer is indicating the front sensor when the rear sensor is bad (and, by extension, vice-versa), or the wiring diagram is wrong.

Has anyone else ever experienced anything like this? Thanks.

Welcome . He probably replaced the wrong O2 sensor the first time. Do you know what the exact code was?

Thanks, Tony.

Here is his note from the first visit:
"Scan test computer, found trouble codes #P0133 and #P1131. Test and confirm bad oxygen sensor. Replace front O2 sensor"

Now here's his note from the second visit:
"Computer had code P0133-B1 S1 slow response. P0131-B1 S1 lean. P1151-B2 S1 lean. Tested oxygen sensors. Found wrong sensor was replaced. Replaced correct faulty oxygen sensor. Cleared codes. All parameters are normal now. Road tested OK."

he didn't charge me for the second sensor replacement (he's a good guy that way), so I got two new sensors for the price of one. His explanation of the reversed codes/wiring diagram was verbal, so I may not have conveyed it as coherently as he did.

"Scan test computer, found trouble codes #P0133 and #P1131. Test and confirm bad oxygen sensor. Replace front O2 sensor"

He changed the Bank 1 O2 sensor the first time as the sensor was tested as bad.

"Computer had code P0133-B1 S1 slow response. P0131-B1 S1 lean. P1151-B2 S1 lean. Tested oxygen sensors. Found wrong sensor was replaced. Replaced correct faulty oxygen sensor. Cleared codes. All parameters are normal now. Road tested OK."

Maybe your Bank 2 O2 sensor was also about to go bad the first time.

It is good that you have a honest mechanic and that you trust will do the right thing.

Note that there could be many reasons for lean codes. Here is a good article to read.
http://www.v8sho.com/SHO/TSB0197HO2SServiceTips.htm

The next time your Check Engine light comes on, take it to Autozone or Advance to read the codes (free service), or buy a reader or scanner from $75 and up. I believe most shops charge $75 or more just to hook up a scanner to read codes.