OK, I'm confused - I thought sensor voltage is supposed to switch back and forth across the magic .45 value, several times each second!

OK, just to add to the mix (no pun intended), A/F ratio sensors are different than O2 sensors, in that they produce a steady voltage, typically in the 2.2 - 4.2 volt range, not a rapidly fluctuating voltage. (Motor, Feb. 2000)

My "primary O2 sensor" is in fact an A/F ratio sensor.

The cold doesn't have anything to do with it o2sersors have a heating element in it espically foreign cars that's what the 4 wires are for , change our o2sensor and it should solve the problem... o car has to be driven about 10miles after insalling it before reading it again so take a little cruise also

Thanks for the response...

I did drive the car for ~ 20 minutes this afternoon, then looked at the two sensors' voltages.

The primary still showed a relatively flat output in the vicinity of .6 volts, the secondary was much more variable, but on average lower than .5 volts. It looked like a heart monitor or sine curve, more what I would expect from an O2 sensor.

The primary is an A/F sensor, heated of course, and based on what I've read about A/F sensors, they should maintain a relatively constant voltage (subject to changing conditions of course), unlike the old school sensors that crossed over several times per second and only averaged a constant voltage.

The CEL has stayed off, hasn't it, and it runs great?

Yes to both questions. I am just trying to understand how these things work, and to optimize my performance/efficiency.

It was running fine with the CEL, though the weather and road conditions were so bad I wasn't driving as vigorously as I normally do.

The CEL went away after I cleared it with the scanner and didn't return. It was only a few days before I replaced the sensor, so maybe it would have returned, maybe not.

It was 80K miles old and only $50, so I figured I might as well replace it.

Well, I couldn't stand it, so I went and ordered a new primary from Rock - best price I could find, $180 shipped. I've already received an offer of $20 shipped for my 80K mile old original primary!

It arrived today, but I wasn't home so FedEx wouldn't leave it in my screen door, even though there was no signature required.

Hoping my performance and fuel economy improve even more with the new primary. If not, preventive maintenance.

Thanks again, txgreasemonkey. Both sensors for ~ $225 (maybe ~ $200, if I sell the original), vs. $600 from the dealership!

Sorry to bring this back from the dead. But the CEL came on again today, same P0420 code.

Random Tech, the manufacturer of the highflow cat I have installed, says this is common because their converters flow better than OEM.

I'm confused - the secondary 02 sensor usually triggers a P0420 if the cat is clogged - which would result in less exhaust flow.

So in one case the code is set from decreased exhaust flow and in the other case from increased exhaust flow. I know the sensor measures the oxygen, but I don't understand how two opposite conditions can result in the same code being set.

Anyone?

Did you ever rule out the other possibilities I suggested back in December?

https://www.askmehelpdesk.com/cars-t...ml#post1444794

No, other than checking the exhaust for damage (I replaced the entire system, head to tailpipe, last summer, and I just finished it off with new OEM A/F primary and O2 secondary).

And, in light of the fact that I installed a highflow cat and it started to throw this code when winter's cold temperatures arrived, and this is a common occurrence with highflow cats... well, it seems to me that the highflow cat is causing the code. Even the manufacturer says it is a common occurrence with their parts.

And - the code was thrown once, back when I started this thread, about one or two months ago. I cleared it, replaced the primary and secondary and it hasn't returned again until today.

Seems those other potential causes would cause the CEL/code to be much more consistent, no?

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Originally Posted by paulkramer

Seems those other potential causes would cause the CEL/code to be much more consistent, no?

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Yes, but the same could be said about the converter. They've all been functioning each time you've driven the car and it's been a month since the last occurence; so why the change?

Good question.

And IDK if a P0420 gets thrown instantaneously, or if the inefficiency has to be detected X no. of times before the CEL comes on...

Not positive on that. I've seen lights get set as soon as a sensor is unplugged, although the light will go out on it's own after the problem has been corrected and the vehicle runs through a certain number of drive cycles.

I believe it varies from code to code as well - some are instantaneous and others are not.

Yeah. I'd say check out those other things, just to be on the safe side. Everyone has varying opinions on highflow converters; some say they're great while others say there's no difference and that they're not worth the money.

Well, they're not OEM, and as such there is always potential for some issue, be it fitment or compatibility with a tightly constrained computer logic.

Strup makes an $18 defouler/extension - provided I don't break something while installing it it seems like a very cheap potential fix.

I notice a difference with the highflow, and with a K20A3 I need all the help I can get.

Price! I paid $400, sold my OEM with 80K miles to a scrapper for $100. A lot cheaper than an OEM catcon... I think dealerships want $800 - $1,100 for the cat for my car.

Ouch! It's ridiculous what they sell a piece of metal for these days. Did the old one need replacement or you just wanted to upgrade?

Well, let's say the desire to upgrade was the driving force...

I had already replaced all the other exhaust components - I got a great deal on a shorty header, then I broke my catback removing it to gain access to the catcon heat shield bolts (heat shield was rotting out and rattling something fierce), so I replaced my catback - two or three times. First one was a Taiwanese junker, second one was that company's attempt at resolving the first one's issues, third I finally ponied up the extra money for a quality exhaust. So I wanted to finish it off, once I realized the cat was now the bottleneck.

I rationalized the expense by telling myself the car had 80K miles and would need a new converter soon - or at least eventually. :p

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Originally Posted by paulkramer

I rationalized the expense by telling myself the car had 80K miles and would need a new converter soon - or at least eventually. :p

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And then you complain when us women want to go shopping ;)