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

[Dirtman]

I know you should replace the upstream O2(s) on a schedule since they degrade over time and will kill your fuel mileage, I did my upstream a while ago but never bothered to do the downstream sensor cause I'm lazy.

The downstream sensor is now 12 years old so I figured lemme check what it's doing on the graph. Voltage was steady but seemed high (.79 volts). Pretty sure normal is around .45?

Should I just replace the dang thing or wait until the computer yells at me to?

 

[sgtsandman]

I would wait for it to yell but that is just me. Last time mine yelled, I had to replace the catalytic to fix the problem. It turned out that there was nothing wrong with the sensor. But that was on a 4.0 SOHC and not a 2.3. So, your mileage may vary.

 

[fastpakr]

I'd wait for it to yell too. Given that its only purpose is to detect converter issues and it doesn't effect how the engine runs, what's the point?

 

[Dirtman]

I'd wait for it to yell too. Given that its only purpose is to detect converter issues and it doesn't effect how the engine runs, what's the point?

Well see I've been doing some homework on that...

Seems originally this was the case, the downstream sensor just checked that the cat was working as was required by OBD2 standards. But as time went on it appears alot of vehicles started using the downstream sensor to make minor fuel trim adjustments as well. The PCM not only uses the sensor to check the health of the cat, but to also keep the cat healthy. The pcm has a target voltage it wants to see from the downstream sensor and if that voltage strays it will adjust the fuel trim to compensate similar to how the upstream sensor operates. Was this around in 2009? Who knows, it seems what exactly is done with the downstream sensor data relating to engine adjustments is proprietary to each company and vehicle and not well published.

I also wonder what the voltage required to set a CEL are. Mine doesn't fluctuate but is high, will the CEL only set if it sees the downstream sensor switching like the upstream, or does it have a set voltage range. I.e if it stays over .8 volts or under .2 volts even if it's steady?

I mean If the target to set a CEL is .8 volts and I'm at .79999 might as well change it lol.

 

[RonD]

0.7 to 0.8v is normal for downstream

O2s detect oxygen
0.1v = high O2
0.9v = low O2
upstream O2 should be approx. .45v showing a good burn and changing fast

Cats should use up most of the O2 when "burning up" exhaust pollutants, so low O2 levels and steady at downstream O2 sensor, i.e. 0.8v

Computer will change fuel mix to Rich to keep Cats hot, and then lean(high O2) to see what downstream O2 does
Sign of a failing Cat is when downstream O2s voltage looks like upstream's, changing fast, so exhaust is not being "burned up" at all
Sign of a failing downstream O2 is harder to detect, steady voltage at 0.6v or lower can be older O2 or failing Cat, hard to say

 

[Dirtman]

0.7 to 0.8v is normal for downstream

O2s detect oxygen
0.1v = high O2
0.9v = low O2
upstream O2 should be approx. .45v showing a good burn and changing fast

Cats should use up most of the O2 when "burning up" exhaust pollutants, so low O2 levels and steady at downstream O2 sensor, i.e. 0.8v

Computer will change fuel mix to Rich to keep Cats hot, and then lean(high O2) to see what downstream O2 does
Sign of a failing Cat is when downstream O2s voltage looks like upstream's, changing fast, so exhaust is not being "burned up" at all
Sign of a failing downstream O2 is harder to detect, steady voltage at 0.6v or lower can be older O2 or failing Cat, hard to say

Ok so either way my downstream sensor is still working since it's not switching (well it does when the engine first starts but the cat isn't hot enough to do anything yet, as soon as the cat heats up it stays steady) and the voltage is in the proper range. I'll let it ride...

For some reason I thought the proper voltage was around .4 to .5 for downstream.

 

[RonD]

No, low oxygen after the Cat is good, means its hot and burning up the O2(with pollutants), so above 0.7v

 

[Orca]

Should I just replace the [downstream O2 sensor] or wait until the computer yells at me to?

Personally, as long as it was not doing anything too weird (like not ever settling down), I would wait, but.... (There's always a "but", isn't there? )

"But" #1: I monitor my vehicles more closely and frequently than the average owner.

"But" #2: There's a chance (IMHO) that the downstream O2 sensor actually does matter for something besides a catalytic converter "monitor".

But as time went on it appears alot of vehicles started using the downstream sensor to make minor fuel trim adjustments as well.

I think you're referring to what's known as "rear fuel trim". I'd first learned about this over a year ago (from someone with a 2004 F150) and had mostly forgotten about it after doing some brief, inconclusive testing to see if my 2004 Ranger supported this. I need to test that more some day.

On a related note: Something that I often forget about myself is the set of public Ford Motor Company (FMC) "yyyy MY OBD System Operation Summary for Gasoline Engines" documents where "yyyy" is, of course, your model year ("MY"). They contain a lot of excellent, detailed information about the so-called "vehicle monitors" that run on OBD2-compliant vehicles. For those unaware, these are the vehicle tests that run on either a continuous or occasional basis, results of which are checked during emissions-compliance testing, but which are generally also useful for basic vehicle diagnostics. Most scantools should report these results.

There is a lightly organized mess of these documents, in reverse chronological order. Or you can simply search using a term like this (with appropriate model year):

• Ford 2004 OBD System Operation Summary for Gasoline Engines

The section called "Catalyst Efficiency Monitor" gets into some things that I think you (@Dirtman) might find enlightening.

That set of FMC documents also contains the necessary information to properly understand the results of the OBD2 Mode $06 ("Request On-Board Monitoring Test Results for Specific Monitored Systems") tests that many scantools are capable of reporting. You've probably encountered Mode $06 data before. I check it monthly, at a minimum, on all my vehicles.

That document for the FMC 2004 gasoline vehicles (including my 2004 Ford Ranger) is 98 pages. There's some good reading in those documents for anyone who wants to get into some of the details.

Hope someone finds this long-winded post useful...

EDIT: Removed request to correct (now-corrected) thread title.

 

[97RangerXLT]

My cel does not have the ability to replace the downstream O2 sensors. Or anything else on the truck for that matter. It will yell about something tho (mine has been griping about the purge solenoid for quite a few years now...)

I would probably wait until it yells and then fix it yourself, because the CEL is lazy and won't do it for you.

AJ

 

[RonD]

My cel does not have the ability to replace the downstream O2 sensors. Or anything else on the truck for that matter. It will yell about something tho (mine has been griping about the purge solenoid for quite a few years now...)

I would probably wait until it yells and then fix it yourself, because the CEL is lazy and won't do it for you.

AJ

And reality doesn't quite make to the surface, but tries and then fails

 

[Dirtman]

My CEL may not fix things but it pays my taxes.

At least I assume it has been for the past 20 years?

 

[Curious Hound]

0.7 to 0.8v is normal for downstream

O2s detect oxygen
0.1v = high O2
0.9v = low O2
upstream O2 should be approx. .45v showing a good burn and changing fast

Cats should use up most of the O2 when "burning up" exhaust pollutants, so low O2 levels and steady at downstream O2 sensor, i.e. 0.8v

Computer will change fuel mix to Rich to keep Cats hot, and then lean(high O2) to see what downstream O2 does
Sign of a failing Cat is when downstream O2s voltage looks like upstream's, changing fast, so exhaust is not being "burned up" at all
Sign of a failing downstream O2 is harder to detect, steady voltage at 0.6v or lower can be older O2 or failing Cat, hard to say

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?

 

[fastpakr]

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.

 

[Curious Hound]

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.

I thought that was the good one.

 

[fastpakr]

Might be?

That's why I want to hear Ron's input. I've been wrong before. Several times today, even.