Do you wait for a CEL to replace a downstream O2?

[Orca]

So, @RonD (or anyone else who knows). This thread started making me wonder about my 97 F150, 4.6l. Bought it used, now gas 150k miles and original O2 sensors. So I just went out to the driveway and started it up, let it warm up while I got the OBDII adapter plugged in and connected to my phone. Here is a screenshot of my O2 sensor voltages from the Torque pro app. If I understand your post, this shows that my cats are nearing end of life and one upstream sensor is getting flaky. Is that correct?

Personally, I wouldn't do anything based on just 1 test but that green line ("Bank 1 Sensor 2") looks somewhat suspicious to my eye. I'd run the test again, after you've driven the vehicle some miles. Even then, there are possibilities of being misled by looking at just one test. The "catalyst efficiency monitor" that I mentioned earlier in this thread could be running, disrupting the data you're analyzing. Or, if "rear fuel trim" is a real thing and at work on these vehicles, it could be causing misleading data. That's why I always recommend lots of testing. If you never see your downstream O2 sensors steadily reporting 0.7 to 0.8 volts, then you should start to be concerned, IMHO.

I don't see anything blatantly wrong with your "upstream" sensors (red and blue lines). They may be getting a little "lethargic", but it's hard to tell and Torque Pro runs on Android, which can cause uncontrollable perturbations in the output of fast-changing PIDs like upstream O2 sensor voltages. Of course, if they're as old as they seem to be, it's probably time for some new ones.

I'm interested in the answer too. The only thing I'd instinctively react to on that image is the Bank 2 downstream sensor holding a near flatline - makes me wonder if the sensor is aging and not switching properly.

The "downstream"/"S2" sensors monitor the catalytic converter output and are not supposed to be "switching". If you start seeing one do that constantly (versus doing it for short durations), over multiple tests, it probably means that your catalytic converter is losing effectiveness, causing the normal, constant, closed-loop, PCM-induced fuel mix alterations (that are quite visible in the "upstream"/"S1" sensors) to start showing up "downstream".

I thought that was the good one.

It is.

 

[RonD]

That's IF there is a B2S2 sensor at all, its only found on Dual exhaust systems, i.e. if there is a "Y" pipe then there is only the one B1S2 for downstream

B1S2 is staying around 0.7v and 0.8v so looks fine, I can't say I understand the time frame, that's not 7 seconds in between samples?
Must be .07 seconds between samples, so pretty steady


The upstreams look OK, but could be a bit on the Lean side, so older
Average is about 3.75v which is good, but............there is nothing to compare it to if both O2s are the same age
Does the MPG seem OK?

 

[Curious Hound]

That's IF there is a B2S2 sensor at all, its only found on Dual exhaust systems, i.e. if there is a "Y" pipe then there is only the one B1S2 for downstream

B1S2 is staying around 0.7v and 0.8v so looks fine, I can't say I understand the time frame, that's not 7 seconds in between samples?
Must be .07 seconds between samples, so pretty steady


The upstreams look OK, but could be a bit on the Lean side, so older
Average is about 3.75v which is good, but............there is nothing to compare it to if both O2s are the same age
Does the MPG seem OK?

Mpg = 15-16 on good days. Not great. But I would only expect up to about 17 or 18bon this truck. Window sticker say it should be 16 city and 21 highway. I've never seen 17 or higher since I bought it used. When towing my Ranger on the car trailer on interstate, it goes down to 8 or 9mpg. I'm not happy with that. But thought it was mostly due to wind resistance.

Just crawled under there. Single exhaust system. But there is definitely an O2 sensor after the Cats on each side before the y-pipe junction.

4 O2 sensors
4 Cats

Dang! This is an expensive exhaust system for a mundane little 4.6l V8.

 

[Orca]

That's IF there is a B2S2 sensor at all, [...]

A good, valid point. But the OBD2 system defines 2 SAE-standard PIDs ($13 for vehicles with up to 2 banks of 4 sensors each; $1D for vehicles with up to 4 banks of 2 sensors each) to determine the presence of all O2 sensors, so if Torque Pro is doing the right thing, it's only reporting on O2 sensors that the vehicle has declared "present".

[...] its only found on Dual exhaust systems, i.e. if there is a "Y" pipe then there is only the one B1S2 for downstream

Maybe I misunderstand here, but I respectfully disagree with this.

I'm not sure exactly what constitutes a "dual exhaust". I have a single tailpipe on my 2004 Ranger, so I consider it "single exhaust". But it does indeed have 4 O2 sensors (all before the 2 post-cat pipes join and exhaust as 1) and the PCM does correctly report that I have 4 sensors (B1S1, B1S2, B2S1, B2S2).

I don't know how a 1997 F150's exhaust is set up, but if Torque Pro reports 4 O2 sensors, I'd make a small wager that there are really 4.

EDIT: I see @ericbphoto edited his post while I was composing the above reply. Four O2 sensors makes sense to me, but 4 cats?!?

 

[Dirtman]

Many vehicles use 3 o2 sensors instead of 4. It's cheaper for the manafacturer and fufills the OBD2 requirements...

 

[Curious Hound]

. Four O2 sensors makes sense to me, but 4 cats

I assume they're cats. 2 small cat-shaped objects on each leg of the y-pipe between manifold and y junction. Maybe one cat and one resonator on each side.

 

[fastpakr]

The second two are probably resonators. Eric - are your 'second cats' before or after the downstream O2 sensors? On the '99 Explorer 5.0 exhaust, the secondary O2 monitors were between the cats and resonators.

 

[Curious Hound]

Sensors are after the cats/resonators.

 

[Orca]

I've been doing a bit of research and I learned something new....

There is something called a "light-off" catalyst, which is "often used to minimize cold-start emissions". IIUC, the term "light-off" seems to refer to the temperature at which the catalyst is effectively converting the noxious gasses.

It seems that the 1997 Ford F-150 may indeed have come with 4 catalytic converters, 2 per bank, as Eric sees. If so, there would be 2 "light-off" catalysts upstream of the 2 "main" or "underbody" catalysts. Even though it doesn't apply to Eric's F150, to be complete, it seems that in some cases there can be just 1 "main"/"underbody" catalyst if the pipes are joined early (i.e. just after the 2 "light-off" catalysts). I suspect that's why the dashed outline is there in the image above.

So a 1997 F150 could very well have 4 cats! Interesting. And probably expensive.

Sensors are after the cats/resonators.

My info says that the downstream O2 sensor "may be located after the light-off catalyst or underbody catalyst".

EDIT: Use equivalent image but with much smaller download size.

 

[Curious Hound]

You're killing me, Smalls. My truck is lowered/leveled. These were not easy to get without jack stands or ramps.


Passenger side downstream sensor. Y junction to the right.

Driver side downstream sensor. Just before pipe turns toward the passenger side for the y junction.

2 passenger side devices.

2 driver side devices.

 

[RonD]

Yes, most single exhaust systems I have come across just have the one B1S2 after the Cats and before muffler
Usually there is one cat on each side of "Y" pipe then one cat on main pipe, then O2, then muffler
The two cats on each side of "Y" pipe is certainly doable and maybe better to keep them hotter, closer to engine, and then an O2 on each set

But in any event if you see 4 then..............there is a B2S2, lol

The graph does make B2S2 voltage look odd though, compared to B1S2

You, for sure, should change the 2 upstream O2s at 150k miles and 24 years old, that should get you 1 or 2MPG

I would also change the 2 downstream just on speculation, 24 years old is old for O2s

 

[Curious Hound]

Everything is "oddball" on this truck. 97 was a weird year. This funky exhaust system, oddball lug pattern on the wheels (5x135? If I recall), and I know I've run into some other odd stuff that I don't remember right now.

 

[1990RangerinSK]

So, @RonD (or anyone else who knows). This thread started making me wonder about my 97 F150, 4.6l. Bought it used, now gas 150k miles and original O2 sensors. So I just went out to the driveway and started it up, let it warm up while I got the OBDII adapter plugged in and connected to my phone. Here is a screenshot of my O2 sensor voltages from the Torque pro app. If I understand your post, this shows that my cats are nearing end of life and one upstream sensor is getting flaky. Is that correct?

View attachment 64394

If your cats are nearing end of life, I would advise calling a veterinarian, rather than posting about it here.

 

[Dirtman]

Everything is "oddball" on this truck. 97 was a weird year. This funky exhaust system, oddball lug pattern on the wheels (5x135? If I recall), and I know I've run into some other odd stuff that I don't remember right now.

Nothing was weirder than those rediculous 7 lug rims...

 

[RonD]

7 is odd not weird, 6 or 8 would be even or weird