Most important Cat. sensor?

[MichaelRS]

I know they are both important in their own way, but between the upstream AF sensor and the downstream O2 sensor is one more important than the other in determining if your catalytic converter monitor will set or not?

After 2 weeks of trying the two major drive cycle patterns for my 2002 Toyota Sienna it's still a no-go on my cat. monitor setting.

I would think my evap system being funky and not setting might have something to do with that, but for the last two times (2 years apart) I was due for a smog check the evap did not set and the Cat. monitor did so I don't know why now that would affect the Cat. now, if it does.

My thought is that my sensors might be a little cruddy and so I was going to replace one or the other of them and somewhere in my head I got the impression that the downstream sensor was the most important. But exactly where that idea came from I can't say. I think I read it somewhere once, but when trying to search that it doesn't particularly come up....which brings me back here.

 

[Passport1]

My feet. She sleeps on them all night.

 

[Imp4]

Many Toyotas of this era are known for a very narrow tolerance band for the allowable sensor readings.
Either you have a bad sensor or two or the cat is shot.
If it were me I'd verify all upstream and downstream sensors are functioning properly. Any replacements id use would be OEM or Denso aftermarket.

If that stuff all checks out or is fixed, I'd look at the cat next and run a cost/benefit analysis to figure if it's worth it to you to take the plunge on a new cat.

 

[vavavroom]

The upstream O2 sensor data allows the computer to adjust the A/F ratio. If this sensor is not functioning properly the A/F ratio will be off and the engine may run poorly and emissions will likely be out of spec. The downstream O2 sensor measures the emissions and triggers a code if the emissions are out of spec but it will not affect the A/F ratio and how the engine runs. Your vehicle won't pass a smog test unless both sensors work properly.

 

[JMJNet]

Did you replace the downstream sensor?
What code does it have exactly?

Different combination of code points to the sensor or cat itself.

 

[MichaelRS]

Did you replace the downstream sensor?
What code does it have exactly?

Different combination of code points to the sensor or cat itself.

I have, and have had, zero codes.

All the other stuff, except my evap, which I know is shot and has been for some time, sets within a few of drive cycles of not even trying.

My cheapy scan tool shows that the Oxygen Sensor Mon. is set and the Oxygen Sensor Htr. is set. Just the cat. and evap monitors remain incomplete. Have replaced nothing as of yet

 

[clinebarger]

Temporarily space the rear O2's, If the Catalyst Monitor now completes.......The Catalyst/s are likely on the ragged edge of setting Catalyst Efficiency DTC's.

 

[GMBoy]

Temporarily space the rear O2's, If the Catalyst Monitor now completes.......The Catalyst/s are likely on the ragged edge of setting Catalyst Efficiency DTC's.

Best advice right here. Can't count how many times I've done this. The P0420/430 codes set after a 10% thresehold is met so that's a pretty tight parameter and why the code is so popular.

 

[Marcozi]

I recommend you get the OBDFusion app for your smartphone or table. It lets you log and export various data streams.

 

[MichaelRS]

Temporarily space the rear O2's, If the Catalyst Monitor now completes.......The Catalyst/s are likely on the ragged edge of setting Catalyst Efficiency DTC's.

How much space and with what do I used to do that with? Is there some sort of extension I would attach to the sensor or the hole it goes in?

 

[brianl703]

How much space and with what do I used to do that with? Is there some sort of extension I would attach to the sensor or the hole it goes in?

A "spark plug antifouler" is commonly used for that. I think you have to modify it so the tip of the oxygen sensor fits through it.

 

[Skippy722]

A "spark plug antifouler" is commonly used for that. I think you have to modify it so the tip of the oxygen sensor fits through it.

Yep, though I don’t think they’ve ever been used to prevent spark plug fouling lol! I’d just drill out the antifouler if the o2 sensor doesn’t fit though.

 

[zzyzzx]

I always replace O2 sensors all at once (meaning all 2 of them at the same time) and have never had an issue with this. I think sometimes people run into issues when they only replace the one bad one and end up with a mismatched pair. Not that I can justify that.

 

[michaelluscher]

How much space and with what do I used to do that with? Is there some sort of extension I would attach to the sensor or the hole it goes in?

Here's me adding one to the post cat O² of Project xB

I forgot to take pictures of it installed

Straight and 90° models are available depending on what kinda space you have to work with

 

[MichaelRS]

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Here's me adding one to the post cat O² of Project xB

I forgot to take pictures of it installed

Straight and 90° models are available depending on what kinda space you have to work with

Thanks very much and thanks to everybody else for their help thus far.

I haven't replaced any of my sensors yet, still trying to figure out if the Upstream one is a AF or O2 sensor, but I guess I'll try that next.

If it doesn't work then I'll go for the spacer thing.
Besides, on a car with over 225k miles on it i guess it doesn't hurt to have new sensors anyway