Addressing some common misnomers of the P0420 code

i got the sensors from Amazon.

Upstream:

$135 Denso 234-9001 Upstream Air Fuel Ratio Oxygen Sensor with 10.6” 4-Wire Harness, for Auto Trans Equipped Vehicles

https://smile.amazon.com/gp/product/B000C5WD2Q/ref=oh_aui_detailpage_o01_s00?ie=UTF8&psc=1
Downstream:

$56 Denso 234-4206 Oxygen Sensor

https://smile.amazon.com/gp/product/B000C5UFOY/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1

I sprayed the nuts on the sensors a day or two ahead of time with Liquid Wrench. The Front o2 sensor came off without issue. was CAKE. was done in 10 mins. The rear o2 was a pita since it had the heat shield still in place. I tried using my articulating socket wrench to get the nuts off but i thought i was rounding the nuts off so i destroyed the shield in order to get clear direct access to the nuts. WELL after i ruined the shield i realized i was not rounding the nuts at all... the nuts were just loosening up really easily! So had i just kept trying then it would have been SO much easier and id still have my heat shield. Once i got the shield off the rest was cake. The downstream o2 sensor does not come with a plug at the end of it. So you have to cut and splice the new sensor in using the original sensor's plug. Im electronically inclined so doing the splice was fun and easy. just remove the entire old sensor all the way up to the plug. Just unplug it from under the floor mat. Then you can easily splice the new sensor to the old plug on your work bench instead of under the truck.

My advice would be to replace the rear sensor then the front if still not resolved. but im also a newb. I also recommend buying an OBDII bluetooth adapter from amazon, super cheap, then buying Torque Pro for your cell fone. Then you can do basic OBDII scantool diag. The rear o2 should show as a solid line with little variation when you hit the accelerator.

 

Do these O2 sensors fit a 4x4? I have an 00 Tacoma SR5 extended cab 3.4L automatic. I replaced the cat last year and the CEL comes off and on after a few months of driving. P0420 code is the only code I'm getting. I'm thinking of just replacing both sensors while I'm under there. Thanks for any help.

 

After 5 months of trying to fix this issue. I could see the volt reading. I could see how the readings responded. I could see how the 2nd sensor would drop below what it should. I tightened the exhaust up. Didn’t fix the issue. However on a cold day I observed a puddle of water under the truck. I could see small amount of water dripping a connection in the exhaust. Couldn’t hear any leaks. But when I put my had up there I could feel it sucking air. New gasket , problem solved. If it wasn’t for the moisture it was sucking in I don’t think I would of found the issue.

 

I've been dealing with a loss of MPG for a long time now... I used to get consistent 18mpg mixed and now it's more like 14-16. For a long time I had the p0420 code (bank 1). I think the longest I had it was a 3 months. Then it would go away come back. But now I haven't seen it in at least 6 months, but my MPG is still shit. I changed plugs and wires and cleaned the MAF and that didn't seem to fix anything. Anyone have any thoughts?

 

Thanks OP for sharing your expertise & insight.

My 2001 Taco has 197K & runs excellent. After replacing both original O2 sensors with OEM Denso's over a year ago (185K), my Taco is now getting p0420 codes. From a troubleshooting standpoint what's the logical next step to eliminate the root cause? Should I be unbolting the Cat and performing a visual inspection to make a decision on buying a new Cat or other?

 

I would feel around for leaks or use a small piece of hose/3/8s in size put it to your ear and put the other around the second O2 sensor and around the connection of the exhaust before sensor while the motor running. Read some of the post again. I saw fuel pump. Maybe it’s a gasket like mine. Cheaper things could be the cause.

 

@tcBob please un-sticky. Too many stickies in the first gen section.

 

Please don't unsticky this...

 

Okay here's my story. 02 dbl cab 3.4 152k Cel came on, said bank 1cat was stopped up. So just replaced rear cat(it was cheapest) cleared cel,15 miles down the road cel back on to late in the day to have code read. What next? Go have code checked again? Replace front cat as well? Replace 02 sensors? Bought truck for my son's first vehicle. Want it right. Please help mechanically inclined but need it in simple terms. Haha

 

Mine came back because it didn’t seal at the front gasket of the 2nd cat which add more fresh air. You can use a small hose an check the connections. I had to use a gasket and grind it down to fit the small gaskets didn’t work. Try the cheaper items first. Clean throttle body. Use C foam.

 

We welded my cat on so no gasket. What is procedure for cleaning throttle body.

 

Finally got mine fixed. Was the rear 02 sensor. 80 bucks sensor and labor. Wasn't that bad at all.

 

That’s good to hear.

 

Can someone explain this sentence? What is meant by the "activity?" Is it measuring CO2 or some other gas? What is "approaches the waveform?" Does that mean they (front and rear sensors) are reading at or close to the same level? or 80% the same? (That is, less than a 20% difference?)

Thank you.

 

Great write up! My buddy just bought a '01 Tacoma farm truck with a 3RZ that produced that code. I told him to take it the the highway and let her eat. After driving it like it was stollen for a week the code has gone away. Cheers!

 

is there a delete mode? where you can straight pipe it and run with out the cel coming on? How do the guy's with headers get past throwing codes. Just wondering, like wunhunglo mine was doing same. replaced lower cat and both sensors. Light went out for about 5 days came back on code says bad cat. No loss of mpg runs good getting about 19mpg. Hate seeing cel on all the time. upper cat is a little pricey but can't see doing it if everything is jiving except cel

 

That’s what mine did I had air getting into the exhaust before 2nd sensor

 

so maybe where top cat is bolted to exhaust it might be leaking? Bottom cat was cut out and welded in at muffler shop.

 

Take a short piece of hose an listen. How I found mine by little drip of water in a place where there should be none

 

So if I am correct in understanding then a p0136 code should not affect mileage? I have to replace the dreaded rear O2 on my 04 with a 2.4. It throws a code within 30 miles everytime I reset it.