My trucks symptoms include a illuminated service engine soon light, sometimes a high rpm idle, and sometimes a rough irratic idle. When a scan was done it indicated the oxygen sensor was defective. The issue is: which specific one (there are multiple oxygen sensors) or how could I check or detemine the faulty one for sure. I am trying to economize and replace only the bad one myself if possible. I own a good multimeter, can it help determine the bad one? Idea's???

 

The scan code should tell you what one it is. Or at least what side.
The front ones do all the work.You don't need to replace any of the back ones.

 

What code was shown, when the scan was done? I would think that would help narrow down which sensor it's complaining about, and narrow down your investigation.

The codes usually differentiate between upstream and downstream sensors.

 

haha. I was too slow I guess. Phil6608 already answered you. :-)

 

My '99 F150 4.2L has 246k miles - keep getting check engine light - I replace the 3 O2 sensors - 6 - 8 weeks later another check engine light w/ same error codes ... anyone have an idea on what's the matter ?
Thx!

 

Shop did not agree with the oxygen sensor diagnostic so they did their own. They found a lean condition on left and right banks and said that indicated a vacuum leak. Couldn't visually find it so I gave the go ahead for a smoke test. There was a hole in a rubber elbow that went into the throttle body area. Ok, now it idles beautifully "unless" I use the AC, if I switch it on it immediately starts to surge then quits if I turn if off. Whatcha think??? Maybe a vacuum line that controls the doors in the ac system??? I didn't discover this until after I left the repair shop.

 

Bighonker, my 97 4.2l is doing the very same thing. Did you ever track down what was causing the engine surge while running the A/C. I've also noticed if I switch the hvac control to "panel" from off, I get a very noticable but temporary surge while going down the highway. Anyone else have any ideas where to look?

 

The IAC is sticking or frozen.
It has to function to control idle under all conditions.
Good luck.