Originally posted by HITMANinMI:
"A downstream O2 sensor CANNOT cause a 0171 code."

Yes it can it did on mine.

He showed me the computer hooked up to it, the upstream was switching from "lean" to "rich" every second, the downstream was stuck on "lean". he said it should be switching because of the different fuel readings, he says no way should it be stuck on lean.

Who told you PO171 cant be the downstream O2?







Nope, you didn't listen, and I read your other post your mechanic both CLEANED the MAF (right) and replaced the downstream sensor (probably wrong)

Why?

Read here: http://www.findarticles.com/cf_dls/m0BQA/2_80/71847178/p3/article.jhtml?term=

Quote:

With the advent of OBD II, all vehicles utilize a heated sensor downstream of the oxidation catalyst to monitor cat efficiency Because a properly functioning catalyst will use up the excess oxygen in the process of converting HC and CO to CO2 and H2O, the oscillations in the rear O2 sensors should be minimal. Note that rear O2 sensors are not FLO. On the Ford models, the heaters on these rear O2 sensors are not turned on until the exhaust system has had a chance to toast out the moisture. This is done to reduce the chance of thermal cracking.



Notice bold part, which is what I told you before. Now if the engine was running lean, that sensor will stay lean, but that downstream sensor WILL NOT switch at the same rate as the upstream if the CAT is working properly.

In fact, if that downstream sensor begins to switch at the same rate, you will get a MIL for CAT efficiency as the PCM will think the CAT is no longer working.

So like I said about a week ago, you DON'T WANT the downstream sensor switching at the same rate as the upstream.

But hey, it's your money, so you can do the research and double check your tech, or you can just write a check, it's up to you.

TB

"Seems like our society is more interested in turning each successive generation into cookie-cutter wankers than anything else." -- Jato 8/24/2004