P0420 code

adriggs1 · Post by **adriggs1** » 02 Dec 2017, 23:33

Hello all, 2001 S60 with 213k on it. I’ve had a nagging P0420 code for a couple years. It used to be on an off with no apparent cause such as a loss in performance, etc. The last couple months it has been on. I read a lot through various forums and found that there are a long list of reasons for this code. This was before the code was constant but my car is hard to start at times and stumbles in the first few seconds. I also have my RPM drop suddenly at 40 MPH or so when coasting or braking. I let off the throttle and the RPM stays for a second then drops to about 1000. It’s almost as if the engine loses power to keep up without throttle. This happened whether the code was present when it first started showing up. My car idles at 750 rpm most of the time. When I push the throttle it runs without a problem. I have replaced over the months the O2 sensors, injectors, spark plugs, air filter, MAF sensor, checked for vacuum leaks and exhaust leaks. My cat doesn’t rattle and exhaust doesn’t smell. The O2 sensors are working properly. I’m appalled as to what it could be. Anyone have a nagging code or advice? Thank you!

adriggs1 · Post by **adriggs1** » 05 Dec 2017, 18:45

I attached a screenshot from the OBD2 reader I have. Shows the freeze frame for my code the car is showing. I do not recall anything out of the ordinary happening as it was highway speed. The idle boost pressure is -10.9 PSI, drops to 9ish when I push the throttle, then jumps up to about -11.7 PSI. I'm suspecting a vacuum leak as it seems there may not be enough of a vacuum. I cannot find one if there is. Any help in interpreting what the issue may be is helpful, thank you!

: Freeze Frame for code

Post by **abscate** » 06 Dec 2017, 03:51

When you clear it, how long does it take to come back? That's often a spurious code from a small vacuum leak or manifold crack, or an exhaust leak.

adriggs1 · Post by **adriggs1** » 06 Dec 2017, 11:49

It usually takes a 20-30 minute drive on the highway for it to confirm the code after I reset it.

Post by **abscate** » 06 Dec 2017, 12:21

The cat efficiency code is triggered by a deviation in the ratio of the two OXS - so if one of them goes bad, you can get that code.
Do you have a code reader which can report OXS voltage?

adriggs1 · Post by **adriggs1** » 06 Dec 2017, 15:52

The front voltage seems to stay from 0.4-0.6 with an occasional dip or spike to lean or rich. The rear sensor stays steady most of the time but does jump from lean to rich more often than the front. I am going to go on a drive once I get home and will try to get a graph of the sensors' voltages. I will provide an update, thank you!

adriggs1 · Post by **adriggs1** » 22 Dec 2017, 13:34

I apologize for the delay; school has been busy. I took my car to a couple local exhaust shops to have my car hooked up and tested. Both shops said the car needed a new cat as it was clogging and quoted about $1,500 for both as well. I can’t afford it now but it’s still cheaper than the dealer by about $300. Thanks for the help!

93SCMax · Post by **93SCMax** » 22 Dec 2017, 14:37

Just a couple hints, assuming you are a poor student...I wouldn't jump on the replacement cat just yet. As abscate suggested, confirm that your rear O2 sensor isn't bad. O2 sensors are definitely cheaper than a new cat. Also, a clogged cat is pretty unusual but something I'd expect an exhaust shop to try to sell you. For a 16 YO car, doesn't make a whole lot of sense to drop that kind of $ for a new cat. If the cat is really your problem, is it possible to go to a junkyard and source a replacement? I see 2001-2006 S60's in the junkyard all the time so you may have a source for a relatively cheap replacement part if you can do the disassembly/assembly.

Which ever way you go, good luck...and let us know the outcome.

Post by **RickHaleParker** » 22 Dec 2017, 15:16

The front voltage seems to stay from 0.4-0.6 with an occasional dip or spike to lean or rich.

Spikes indicate the O2 sensor is on its last leg. Yours is new it should not be spiking. I would be looking for something upstream of the O2 sensor ... vacuum leak, intake leak a dirty mass-air-flow, something tripping up the metered air measurement.

From your done list, it looks like intake leak is one you have not checked. Spray starting fluid around the intake, if there is a leak. The leak will suck in the starting fluid and your hear the engine surge.

Front going from rich to lean, rear steady indicates the cat is working.

If the cat is really your problem, is it possible to go to a junkyard and source a replacement?

Not in the United States where it is illegal to sell a used cat to anybody not in the recycling chain.

93SCMax · Post by **93SCMax** » 22 Dec 2017, 16:48

RickHaleParker wrote: ↑22 Dec 2017, 15:16 If the cat is really your problem, is it possible to go to a junkyard and source a replacement?

Not in the United States where it is illegal to sell a used cat to anybody not in the recycling chain.

I didn't know that. If I misspoke, I apologize. I've never had to replace a cat, and I've owned a lot of older cars, but the junkyards I deal with (in SC) will sell you anything on the car. I've bought a used exhaust, but I don't remember if the cat was part of the system.
Anyway, a clogged cat isn't a common failure so I'd focus on the o2 sensors and any potential upstream leak. Just trying to save the kid some $$ and hoping the exhaust shops aren't trying to take advantage of him.

Thanks for the input RickHaleParker.