Hi all,

On my way home from a road trip this weekend, I pulled into a drive thru for a burger, and noticed that my O2 sensor was fluctuating a lot while I was in park. I do have my engine light on for the P0420 code, which I pretty much am just ignoring for now, but wondering if this could be related. I'm going to replace that sensor but is there another reason it would show that data?

Also, you may notice the high trans temp. I had just climbed a hill, but while I was in line at the drive thru, it didn't really go down much if at all. Could this be related? Probably time to flush my fluid cause it's pretty dark, but I've heard some people say it could lead to other problems if my trans isn't in good health. Thoughts?

Thanks in advance

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Does your ‘99 have the “federal” emissions (one catalytic converter) or “California” emissions (two catalytic converters)? I’m thinking you have the California emissions based on your Bank 1 O2 readings.


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You're thinking right. California, dual cat. Forgot to mention that, thank you.

....Beuller...?

420 can also indicate exhaust system leaks. It pops up on mine 2 or 3 times a
year. I just clear it and party on.

How's your fan clutch? If it's worn out, it can cause the heat problem you're seeing.

I think your scan tool has switched sensor 1 and sensor 2 voltages. Normally sensor 1 voltage fluctuates as it senses and adjusts fuel trims and sensor 2 voltage should stay steady if your catalytic converter is operating as designed. Even if your catalytic converter is bad you should still see sensor 1 voltage fluctuating.

Yeah I'm not super worried about the 420 code, it comes on and off and I think it's mostly related to my driving style in the summer - low speeds, high ambient heat. I've checked for exhaust leaks before and found nothing.

I can check the fan clutch to see how it's functioning. Would that cause my trans to be hot but not my engine? I would think I'd see higher temps for both engine and trans if the fan clutch was bad. But I'll give it a look this evening.

I have been wondering if they were switched as well, because they show opposite what I would expect. I don't smell anything that would indicate my cats are bad, and I just passed smog by a significant amount.

What would you recommend I do about the sensors?

I've seen reports of high trans temps from people rock crawling at slow speeds
in too-high gears. The motor is fine, the trans is getting hot. The remedy is
downshifting to speed up the engine = fan. Something similar might be going
on here if the fan clutch is slipping, not enough air movement at idle.

Nothing. The problem is your scan tool not the sensors.

"Normal" O2 sensors swing wildly like that - its how they work. Since you have a CA emissions front AFR sensor (not an O2 sensor), it won't swing wildly like the rear O2 sensor will/should. No worries there.

The trans temp isn't crazy high, but may be something to keep an eye on. The official Toyota way to cool off the trans is to idle in Park. Drive won't allow it to cool off as fast. Driving with the converter locked at highway speeds will also keep the trans cool, relatively. Heat is mostly created in the torque converter as it slips.

-Charlie

Thanks for the info Charlie! I was idling in park for probably 15 minutes in the drive thru, with the heat blasting, and the trans temp didn't drop at all. Does this indicate a problem? I think I'll swap the trans fluid this weekend since it looks pretty burnt.

@prybar . Maybe I missed it. When was the time your transmission filter screen was changed? If your filter is plugging, you will lose fluid volume and thus temperature increasing due to the lack of less fluid . May not be the case. Just a thought.

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Probably never been changed, if I had to guess. I'll give the filter a look when I do the fluid flush. Hopefully it doesn't look too bad...

Thanks for the tip!

Looks like it peaked at 220*F, based on the image above, so I wouldn't worry too much about it right now. Still worth a drain and fill or two if the fluid isn't looking 100%.

The temp reported by the ECU is the HIGHEST temp the fluid ever gets - the sensor is on the outlet to the OEM cooler. The pan will be cooler than what is reported by the ECU.

-Charlie