So I got a P0420 code which is "catalyst system efficiency below threshold (bank 1)". Which could be the catalytic converter (hopefully not) or as many have suggested the oxygen sensor.

I am not expert mechanic but have some basic mechanical skills and a willingness to try. This seems like a job I could tackle myself.

I have a 2000 4Runner SR5 V6 with a manual transmission. It has 145K miles and I am the original owner. I have the original O2 sensors, catalytic converter. In fact everything in the exhaust system is original.

I have read around this forum and elsewhere but still have a few questions.

1) Sometimes the sensor upstream of the cat is called an oxygen sensor and sometime and Air Fuel sensor. Which is it and which do I replace?

2) In looking under my 4Runner I see what looks to be two catalytic converters with one sensor before the first and one after the second. Does 2 cats mean I need a cali sensor? Seems unlikely as it was bought in New Hampshire new.

3) The recommendation seems to be to avoid Bosch and use Denso or OEM. Local parts shop has NTK. Any opinion on those.

4) Am I barking up the wrong tree with replacing the O2 sensor? The P0420 first came on after a 4000 ft climb and descent of Mt Washington. I hoped it was due to the unusual driving of that day and sure enough the CEL did not come back on for another 6 months. I turned it off again and it came back on a month later.

Thanks

 

I had the same code on my 01 tacoma 4x4. Before you start putting money into expensive oxygen sensors check a few things out first. First look for exaust leaks The o2 sensor that's probably throwing the code is the one right before the cat conv start at the exhaust manifold to connector pipe then the cat. Any leak will cause the code. Pay attention to the area around the sensor. That rusted out on mine. Also try removing the sensor testing it.

 

Lots of these California spec vehicles on the East Coast.

Yes you need the California emissions parts.

I have heard it is possible to change them back to federal specs but requires a ECU change lots of work and expense .

 

Two cats means you have California emissions. Lots of Cali. trucks were sold all over the country in 1999 and 2000; many states adopted the Cali. standard before it went Federal in 2001. My bought in NJ '99 is a Cali.

Cali. emissions also means your front sensor is a wideband air/fuel sensor. Note that ALL 2001 and later 3rd gens have the two-cat system as well.

The P0420 causes are exhaust leaks, rear sensor, front sensor and the cat itself. At 145k, your sensors are past normal life; mine went at 90k (front) and 110k(rear). The rear ones are usually responsible for a P0420; the front ones usually die first. The front sensor can also degrade performance and fuel economy as it ages. The rear sensor has no effect on performance.

If you have the money and plan to keep the truck, I would do both sensors. If you just hope to clear the code for minimum $$, first rule out exhaust leaks and then do just the rear one. Changing my rear sensor at 110k cured my P0420; I'm at 160k now.

Finally, forget changing back to 49-state. It's illegal under federal law and the states run the VIN and will expect it to perform to the Cali. standard it was built to. The new ECU will report 49-state and your inspecting authority computer will nail you. Unless you don't have inspections, in which case you don't need to worry anyway.

There are also simulators sold by URD that can make the problem go away in an illegal and ecologically impure way.

 

Durk your posts are most helpful, I have the P0420 code and the P0442 (exhaust leak) code. I think I have a line on what to do w/replacing the rear O2 sensor, I will troll the site and look for the other code. Already checked the gas cap. I plan to look around at the exhaust hoses..any advice appreciated.

Thanks

 

P0442 has nothing to do with the exhaust system. It refers to the evaporative emissions control system, which deals with vapor from the fuel tank. I've never had to attack that one, so I'd start here:

http://tacoma.site40.net/4Runner_96-...fe/cip0440.pdf

 

Thanks I'll post again w/ update

 

Based on comments here and in other places, I'm going to replace the rear O2 sensor to attempt to solve the P0420, but I'm still confused.
If the correct behavior is that the forward sensor outputs a sine wave on normal engine performance, and the rear sensor outputs a flat line after the exhaust is treated by the converters, what's the fail look like?
Is it just the wrong voltage level? I just can't see how the sensor can output anything but a flat line if it's failing??

thanks

 

At that mileage you are better of investing money into a O2 simulator, because it will not be long after you replace the O2 sensor will you need to replace the cat converter to keep the ECU happy. A new O2 with old cat will still throw codes.

 

Thanks, I'll look into that.
but even though the truck is 13 years old, it only has 121000 miles.
What makes the converter(s) fail?

 

Old age makes them fail. Their performance degrades over the years to the point when the ECU is not happy anymore (and the EPA is not happy). There's physical failure of course too, rust, fouling, honeycomb damage.

 

The cats arent cheap. Mine went a few years ago and I brought it to the dealer. It was something crazy like$5000. Local guy charged me $900 for a generic cat. My check engine came on again, both sensors had been changed. I got the simulatot and havent looked back. I love my truck, but it is 11 yrs old.

 

Mine keeps popping up periodically, i think i ended up finding a small leak at the flange where the cat connects to the exhaust manifiold like a few others mentioned, i can only feel a little exhaust escaping there but probably enough to make mine come on from time to time.

 

It's been a while but I replaced the O2 sensors and had the codes (420 and 422) cleared so the check engine light went out. After a few days the light came back on, suspicious since the temp went from 70 to 40sh a few nights. Whether it's related or not I'm not sure. About a week later the when warmer temps came back the check engine light extinguished and has stayed off since then (early April).

Durk - I did print out that troubleshooting guide but could only go so far, voltages were good etc...appreciate the help.

 

I changed the spark plugs and wires before I tackled the O2 sensor, and I found a LOOSE spark plug! Obviously I'm not sure how long it's been loose, but there was blow-by.
After a couple of days, the CEL went off. I reset the computer, and the light has not come back now in 3 weeks.

It looks like there was enough getting into the exhaust to overwhelm the converter and cause readings from the O2 sensors to set the P0420 code. Luckily the sensor I bought is in a sealed bag, so it's obviously unused and returnable...

 

That's good, at least the spark plug fix was probably cheaper than the O2 sensors..

I'll keep that in mind if this happens again.