Oxygen Sensor After Cat Voltage Toggle - TCCoA Forums
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post #1 of 3 (permalink) Old 11-25-2007, 02:12 AM Thread Starter
Join Date: Oct 2007
Location: IL
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Oxygen Sensor After Cat Voltage Toggle


1) Is my bank 1 sensor 2 dirty or going bad?
2) Is my catalytic convertor going bad?


I have been working on my 94 Tbird 4.6L for several months trying to find out why the MIL intermittently comes on and the engine performs more like a 4-cylinder than a V8. I purchased a ProScan Universal Scan Tool w/USB Interface, ran many diagnostics and read many threads on this forum (this is one great forum). I have cleaned the MAF sensor twice (second time I lightly scrubbed the sensing element with a Q-tip to remove all contamination) with contact cleaner 64-4315 and then flushed with 99% Isopropyl alcohol. Also, corrected a couple of vacuum leaks with the hoses around the PCV and vapor canister. The spark plugs were replaced early on since the first code thrown was a P1131 and it was obvious from checking the spark plugs that bank 1 was running lean and bank 2 was running rich with heavy carbon deposits on the plugs. I test drove the car for about 30-minutes this morning and monitored the long-term fuel trim, since the last codes thrown were a P0171 & P0174 and the freeze frame data showed the long-term fuel trims at 25%. With the MAF perfectly clean and the hose clamp tight on the inlet to the MAF housing (accidently left it loose from the first cleaning) the Tbird flew very well for an engine with 117K miles on it. The long-term fuel trim varied only +/- a few percent during the 30 min run of some heavy acceleration, but mostly light throttle 60 MPH country road driving. Once again, I think I have the problems solved.

But do I? I graphed the oxygen sensor voltages while sitting in my driveway at idle after the 30 minute run just as a final check. I graphed each one individually and notice that bank 1 sensor 2 was struggling to toggle at idle. The other three sensors toggled as expected. I graphed bank 2 sensor 1 and sensor 2 at the same time and sensor 2 was toggling just like sensor 1 with just a slight phase delay. I graphed bank 1 sensor 1 and sensor 2 at the same time and sensor 2 was toggling about every 5th time that sensor 1 toggled. If I increase the engine RPM to about 1000 then, sensor 2 started working just fine with the same toggle as sensor 1 with the slight phase delay. Should this be a concern? I have never had the MIL come on while idling.

Is bank 1 sensor 2 dirty or going bad?
Is the catalytic convertor going bad?

Sorry for the long posting, but since I have already used a lot of the advice previously posted, I wanted to let anyone willing to help know what has been done.
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post #2 of 3 (permalink) Old 11-25-2007, 08:32 AM
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You may know this already but the sensors after the cats (sensor 2, on banks 1 and 2) are supposed to switch slower than the primary sensors. The ones forward of the cats (sensor 1) are what the EEC uses to adjust fuel trim and such, while the ones after the cats are there only to serve as catalyst monitors. With properly functioning cats they are supposed to switch slowly, however at idle it may be that your bank 2 cat is just not hot enough to burn off the excess hydrocarbons- thereby explaining why when you increase the engine speed, it begins to function normally again. I wouldn't worry about a bad sensor, usually when they go bad they freeze at one reading or switch extremely slow and within a narrow range, so it's probably that #2 cat just getting a little worn out. My factory cats lasted 300,000 miles, and were fine right up to the day I pulled them off, so don't sweat it.
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post #3 of 3 (permalink) Old 11-25-2007, 12:39 PM
No, Mr. Lemmywinks, No!!!

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I would replace both front sensors, and swap the two that toggled the best on the rear locations.

I had some issues/ codes this summer, and even without throwing a code, the sensors were making my car run poorly and get crappy mileage.

My 10 year old sensors were not quite bad, but replacing them made a major difference.

The rear ones can affect the operation as well, but IMO only if they short and take the whole circuit for that side down. (That does throw a code)

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