OK, this isn't for a Ford truck, but it's a pretty generic question (and I'm not a member of a Buick Sedan Enthusiast forum). My son's '98 Buick LeSabre was setting codes PO131 (O2 low voltage) and PO171 (system too lean). I replaced the O2 sensor and I'm still seeing the PO171 (lean) and also a PO133 (O2 slow response). Any guesses what else it might be? I could see a plugged injector or something giving a lean condition, but why would a new O2 sensor be giving a slow response code?

 

Bumping up to the top. Anyone have any suggestions?

 

My thought is the PCM sees the one bank lean, and is trying to compensate for it but it may be fighting an uphill battle. Whether the lean condition is due to a clogged injector or intake leak I don't know but either way the system may not be able to dump enough fuel in to overcome the lean condition, or at least not as fast as the PCM thinks it should be able to.

Of course there's always a chance the new HEGO is faulty. Can you swap HEGO's left to right, reset codes, and see if the code follows the sensor? This only works if there's 2 banks, I'm not familiar with that engine though so maybe this idea won't work.

I use the $5 TorquePro app on my Android phone with a $15 OBDII bluetooth adapter for reading codes on my 96'. I can plot sensor voltages over time. I replaced bank 2 HEGO but not bank 1, you can see in the graph that the new sensor cycles rich/lean at a higher frequency, and the amplitude of the wave is greater. This triggered a slow response code for bank 1 but it was obvious why once I knew exactly what the voltages were doing.

 

There's only 1 O2 sensor upstream of the cat. It's in the Y-pipe after the flow from the 2 banks have merged. There's another O2 sensor farther down the exhaust system, but I'm pretty sure (and the infallible internet has confirmed) that I've got the right sensor.

I've thought about an exhaust leak making it look like it's lean when it isn't, but I don't hear any ticking or other exhaust noises, so that doesn't seem likely.

And I've thought about a sticking injector(s) or low fuel pressure or intake leak or something making it actually run lean (to the point that the computer ran out of range to correct for it). But that doesn't seem to make sense to me why it gave a "O2 slow response" after putting a new O2 sensor in it.

That might point to the new O2 sensor being bad too, but I really hate throwing more $60 parts at it in hopes of getting it fixed.


I don't have a smart phone (no cell phone at all, I'm really a dinosaur!). All I have is a cheap code reader. It tells me the codes and lets me clear them, but nothing more. But I might have to get my kid to put something like that on his phone to try to look at the trace like you're talking about. I can at least put the old O2 sensor back in and compare its signal to the new one.

 

My dad is a land line only kind of guy, and he gave up on the internet due to his frustration with dial up years ago. So you're still tech savvy compared to some. I have a love/hate relationship with these phones. So many devices in one but it can be a massive time waste..of course that is no one's fault but my own.

How do the spark plugs look? If they're black it'll at least suggest it's getting fuel. I agree that swapping in $60 parts on a whim is not a good route. If you can see the voltage traces between the two sensors that would be a start. It's also useful to see if the voltage pegs high under wide open throttle (rich).

Does the car run okay?

Edit: after replacing the sensor, did you reset the PCM (wipe the codes), and then the code was triggered again?

 

I'm no Luddite. I use computers all the time at work (I'm a mechanical engineer). And we upgraded from dial-up years ago! (like 3 years ago). I just like not having to feel "connected" all the time.

I don't have ready access to the car. It's my son's so I only see it when he comes by (which is often when he needs help with his car!). I haven't pulled plugs yet, but yes, that's something I should have already checked.

Yes I did clear codes on it. I did that the first time I looked at it (PO131, PO171 and PO300 I think), then again a few days later before I replaced the O2 sensor (PO131, and PO171) and then a couple days later (PO171 and PO133). I haven't seen it since and don't know if it's reset the codes, but I'd guess it will.

Generally it runs fine. He says sometimes it doesn't make power for a few blocks when first starting out cold. And a couple weeks ago it wouldn't make any power on a long freeway drive (he couldn't get over 60 mph on flat ground). When he shut it off and restarted it was fine for a while and then came back. After another shut-down it ran fine the rest of the trip home and ever since (I'm guessing that was when the PO300, random multiple cylinder misfire, was set). For now he's just driving it.