OBD-II Related Engine Code - P0420

[GaryE30]

I have done a good bit of research about this code and the possible issues causing it and possible fixes. It appears that many people throw this code and generally don't know the solution because there are a few different things that could trigger it such as O2 sensors, catalytic converter, coolant temp sensor, etc. Many of these things aren't cheap and could rack up a hefty bill even if you do the work yourself.

For a quick reference please see the provided link: http://www.obd-codes.com/p0420

I just replaced the post-cat O2 sensor because I read about that one being the culprit the most on these forums and other forums. I had the code cleared and then ran the car for a bit and the light came on. Something is still triggering the code, and before I go and spend more money on parts I wanted to see what is really going on and get some tips from those who may know.

I used a friends OBDII scanner to read the live data coming from the car and this is what I got as far as oxygen sensor performance.

Oxygen Sensor / Bank 1 / Sensor 1 = 0.15 - 0.95 V fluctuating
Oxygen Sensor / Bank 1 / Sensor 2 = 0.92 - 0.98 V fluctuating

*****This data shouldn't trigger the P0420 code, but I wanted to include it for comparison.*****
Oxygen Sensor / Bank 2 / Sensor 1 = 0.13 - 0.96 V fluctuating
Oxygen Sensor / Bank 2 / Sensor 2 = 0.94 - 0.96 V fluctuating


What I interpret from this is that all of the oxygen sensors are functioning properly since the front 2 are fluctuating at a high range and the rear 2 are fluctuating at a very small range.
From some other research (not sure how accurate) I have found that voltages less than 0.3V indicate running "Lean" and voltages greater than 0.8V indicate running "Rich". It appears that my vehicle is throwing a code because the rear 2 oxygen sensors appear as "Rich" which means the catalytic converter is not functioning properly.

However, if this were true, why would my car not throw a P0430 code as well for the Bank 2?

Thanks for any input on the matter.

[jlevie]

P0420 is Catalyst Low Efficiency Bank 1. The causes of that can be a bad post cat O2 sensor, a bad pre-cat O2 sensor, anything that upsets the A/F ratio, or a bad catalytic converter.

The post-cat sensors are in a pretty benign environment and rarely fail (or cause this code). The more likely cause is an upset in the A/F ratio. That can be from an aged pre-cat sensor (their useful life is not more than 100k), intake leaks, a bad CCV, or a fuel delivery problem. Misfres can also cause that code.

What I'd suggest is to replace both pre-cat sensors if they have 100k on them. Then have a smoke test run on the intake and a crankcase pressure check run (diagnostic for a failed CCV). And I'd suggest checking the plug wells for oil from leaking valve cover gaskets, which can cause misfires. Depending on the result of those actions further diagnostics may be indicated.

[r3d e30]

Is this for an E30? E30's were obd I unless it has been converted. Its possible the post O2 sensor is bad, but I would check the cat and and the fuel system to make sure nothing else is wrong that could have caused the O2 sensor to go bad.

[matt]

Free tip: On MS41 a couple of WOT runs will reset that code 80% of the time.

[SpecM]

ummmm what kind of car?

[GaryE30]

Sorry, car is a 1998 M3 with S52 engine. It appears from this scanner that the O2 sensors are functioning does it not? The pre-cat sensors fluctuate as they should and the post-cat sensors are pretty steady, just on the high side.

I have done a simple "carb cleaner spray" leak test and haven't seen any issues. I have read that aftermarket cats can cause this code to occur even if they're functioning. I don't know if the ones on the car are original or not. I am going to look into that.

[GaryE30]

Free tip: On MS41 a couple of WOT runs will reset that code 80% of the time.

Not familiar with MS41. Is that MegaSquirt?

[matt]

Not familiar with MS41. Is that MegaSquirt?

It's the ECU running the car you're asking about.

[SpecM]

Free tip: On MS41 a couple of WOT runs will reset that code 80% of the time.

aka an Italian tune-up

[jlevie]

Sorry, car is a 1998 M3 with S52 engine. It appears from this scanner that the O2 sensors are functioning does it not? The pre-cat sensors fluctuate as they should and the post-cat sensors are pretty steady, just on the high side.

All that check says is that the O2 sensors aren't dead. You'd have to pull them and test them on a mixed gas flow bench to know if they were working properly.

I have done a simple "carb cleaner spray" leak test and haven't seen any issues.

I'm sorry, but that only tells you that you don't have any gross leaks at the places that can be tested with carb cleaner. It says essentially nothing about the presence of other intake leaks. And it doesn't say anything about the CCV.

[GaryE30]

It's the ECU running the car you're asking about.

Gotcha thanks :)

All that check says is that the O2 sensors aren't dead. You'd have to pull them and test them on a mixed gas flow bench to know if they were working properly.

I'm sorry, but that only tells you that you don't have any gross leaks at the places that can be tested with carb cleaner. It says essentially nothing about the presence of other intake leaks. And it doesn't say anything about the CCV.

Yeah, I'll look into the CCV stuff. I haven't messed with that at all on this car. May have to buy another O2 sensor too.

[jlevie]

Your 98 M3 has two pre-cat sensors. If they qualify on mileage, replace both. Then have the intake and CCV tested.

[GaryE30]

I know, I already bought one for the post-cat bank 1. To pass this emissions thing I'd only need the pre-cat bank 1. Right now I don't want to dump $400 on O2 sensors and still have issues. I'm going to take it one step at a time.

Also, this code has been "ON" since I bought the car 2 years ago. I didn't live in a state that required emissions testing and it hasn't had any adverse affects so it didn't stress me too much at the time. The WOT doesn't do anything for me.

[Farbin Kaiber]

In California, on any vehicle, we would replace the cat with this code. Fixes it 98& of the time.

[Kershaw]

my rx8 throws this code. but i removed the cat pipe with the o2 sensor. :D

didnt know codes were standardized.