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Old 09-25-2023, 11:33 AM #1
evamoly evamoly is offline
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Low cat temps

Hi All, starting a fresh thread with this question in the hopes of getting more eyes one it.

A P0420 code brought to my attention that I might have a cat issue, so using an infrared thermometer I registered the following temperatures off the in/out of my front and rear cats after a ~15min freeway and surface street drive:

Front: 490/580
Rear: 520/520

My thought is that I damaged the cats when I was stuck in the desert with a dead fuel injector and limped to the nearest town. But wouldnt the front cat die before the rear if I was sending raw fuel down the exhaust?

In any case, the engine seems to be running well now, and I dont have smog until next year. Any reason to address this immediately?
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Old 09-25-2023, 06:42 PM #2
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If the front cat is doing a good job, there's nothing left for the rear cat to do. No temp difference there doesn't seem like a big deal. P0420 can be caused by other issues too (a few other threads on the first page include cleaning the MAF as a successful solution that a few have had). Don't assume the cat is bad until other options have been eliminated.

How old are the O2 sensors? Is the MAF clean? When you replaced/fixed the injectors, what was the source?

-Charlie
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Old 09-25-2023, 08:14 PM #3
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The A/F sensor was installed about 55k miles back by the PO. O2 is factory to my knowledge. I just cleaned the MAF, with the only discernible effect being that a warm start is smoother. Do you mean the parts source of the injector or the source of the issue that caused me to replace them? I was on a road trip in August and had one injector installed at a dealership which fixed a misfire, Misfire returned in October, on a different cylinder this time, replaced all injectors with Denso from Motor West (think there were three different brands in there when I pulled them out) and did dual ground plugs and wires at the same time. All had been well with fuel delivery/combustion since then from what I can tell.

The temperatures had me skipping past faulty O2 as a potential source of the code...thought that was a dead giveaway for cat issues. But if those temperatures seem close enough then I'm going to revisit the O2. Having read so many accounts of those going bad and throwing the P0420 does seem like a red flag. The code hasn't returned since I cleared 60 miles ago, so I might let it ride until if/when it does. Happy to have this forum as such a great resource to read about these things and ask questions!
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Old 09-26-2023, 07:08 PM #4
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I wouldn't replace the front one again especially if it's the 3x more expensive Air/Fuel ratio sensor that the newer CA emissions and 01-02 4Runner had. However it's very possible that your rear O2 sensor has quit, especially if it is original. Look into a replacement at DENSO Auto Parts | Home and look up the specific part number. Normally Amazon has the lowest price and should only be $50-$60. Much much cheaper than a new cat and super easy to install assuming you don't have rust.

This is probably the most straightforward check engine light to address. It's either an exhaust leak at the cat and the rear O2 sees too much oxygen, the rear O2 sensor has failed or the cat has failed. Or if you're lucky like the guy who posted up lasted week, there's a 0.00001% chance it's a dirty MAF. Worth a shot cleaning it I suppose.
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Old 09-27-2023, 01:13 PM #5
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Thanks for the response. I'm trying to gain clarity on the the live A/F and O2 readings:

S1 reads steadily at .655, S2 fluctuates between .1-.8

Short term fuel trim is pretty close to zero. Any chance that my code reader just has the S1/S2 reversed and theres not actually anything wrong here?
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Old 09-27-2023, 03:53 PM #6
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Quote:
Originally Posted by evamoly View Post
Thanks for the response. I'm trying to gain clarity on the the live A/F and O2 readings:

S1 reads steadily at .655, S2 fluctuates between .1-.8

Short term fuel trim is pretty close to zero. Any chance that my code reader just has the S1/S2 reversed and theres not actually anything wrong here?
The upstream "O2 Sensor" value is a re-scaled version of the AFR sensor output, so it won't fluctuate much. Your results are generally as expected.

The rear/second O2 sensor should have a lag/filtered effect compared to a normal front O2 sensor, but you can't make that easy comparison with an AFR sensor.

-Charlie
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Old 09-27-2023, 08:12 PM #7
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Quote:
Originally Posted by evamoly View Post
Thanks for the response. I'm trying to gain clarity on the the live A/F and O2 readings:

S1 reads steadily at .655, S2 fluctuates between .1-.8

Short term fuel trim is pretty close to zero. Any chance that my code reader just has the S1/S2 reversed and theres not actually anything wrong here?
Looks like everything is good. Toyota has their A/F sensor work on a proprietary 0-1V signal compared to every other manufacturer that used 0-5V. I would look elsewhere for the problem.
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Old 09-27-2023, 09:38 PM #8
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Looks like everything is good. Toyota has their A/F sensor work on a proprietary 0-1V signal compared to every other manufacturer that used 0-5V. I would look elsewhere for the problem.

Not to add their Knock Sensor and Missfire sensing outright sucks!!!!!


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