Newbie question re:o2 sensor replacement

[redgar99]

Howdy, longtime reader, first time poster. I have a 2008 Highlander throwing P0138 and P2195. o2 replacement looks within reach for my capability. Plus this would give me an excuse to expand my tool collection.

I think I've read most of the FAQs, but still have a few basic questions if anyone here has a moment to advise me:
- Since there's no Haynes manual on the 2nd gen Highlanders, is the repair manual from TIS worth the subscription? i.e. will it tell me which sensors/banks are which?
- Will the unversal o2 sensors function both as a plain o2 sensor and as a air/fuel mix o2 sensor, or should I be looking for separate parts?

I've read that this combination of codes could be sign of greater emissions system issues, but I believe replacement of these two sensors is the first logical step. Is there any reason not to proceed with this?

TIA for any inputs and good day.

[sweeneyp]

PM sent
Edit: As AVConsult said below, it is very unlikely they both failed. I've only had 1 fail in a 1997 Grand Cherokee, no other car I have ever owned had a O2 sensor fail, let alone 2 at once.

[AVConsult]

Sweeney probably got you first, but both sensors failing simultaneously is *very* unlikely, especially on a newer vehicle, and almost always points to something less obvious (fuel delivery, air leaks, MAF sensor....)

[redgar99]

Originally Posted by AVConsult

Sweeney probably got you first, but both sensors failing simultaneously is *very* unlikely, especially on a newer vehicle, and almost always points to something less obvious (fuel delivery, air leaks, MAF sensor....)

Thanks for the feedback guys. I should have added that I recently found out that the air filter has not been changed in a LOOOONG time (my wife takes this car to a different shop near her work, and we each thought that was being taken care of). It takes a big man to admit this on a gearhead message board, doesn't it?

When you mentioned the MAF sensor, it made me think of that air filter. But wouldn't there be other codes associated with those other subsystems? Curious how those issues could manifest themselves as basically junk voltage levels on emissions sensors.

Thanks again

[AVConsult]

The downstream (after cat ) is pegged "lean" and the upstream (before cat) is pegged rich. Of course both can't be correct.

I *can* muse that the upstream sensor may be whacked and reporting rich, making the ECU lean out the mixture, which the after cat sensor reports accurately as chronically "lean".

A scan tool would tell a lot more, including whether forced enrichment (like propane) or leaning (pull a larger vacuum hose) makes either sensor respond.

Some free things to try would be measuring the heater terminals--harness cable disconnected) on the A/F sensor (1.5 to 3 ohms or so as I remember), attaching a scope to the heater wires at the A/F sensor (a square wave toggling between 0 and 12v) and pulling the sensor to see if contaminated or ports plugged up.

You can probe wire by using straight pins or safety pins. Make sure the pins don't touch each other or anything metal on engine/manifold. Put a dab of fingernail polish or rubber cement where you pierced the cable.

[AVConsult]

Do check the MAF for contamination, although a dirty MAF usually underreports air flow which causes off idle stumbles, hesitation, etc as the A/F sensor is slower to pick up the temporary lean condition.