help EGR/Cat Conv problem(s)

[jackj]

CEL came on and both codes came up. I was going to try cleaning the EGR and associated tubes connecting to it to deal with the P0401, but was wondering if the fact that I got the P0420 at the same time might indicate specifically what the problem is with P0401, or vice versa?
The printout from autozone says, for P0420, Catalyst eficiency low-bank 1, and under Explanation: The oxygen sensors monitor the catalytic converters ability to store oxygen
The most likely cause of P0420 is:
1 - Cat Conv defective (possibly due to #'s 2, 3 or 4)
2 - Engine misfire or running condition
3 - large vacuum leak
4 - Engine oil leakage into exhaust-valve guide seals, piston rings

Re: P0401, it's listed as EGR system fault; Explanation: The ECM has detected too little flow on the EGR system. Probable causes:
1 - Blocked passage on EGR system
2 - Failed EGR valve
3 - EGR solenoid faulty
4 - Failed EGR pressure sensor where apllicable
5 - Failed EGR position sensor where applicable

UPDATE: The CE light has now gone off; unsure if the codes are gone, too, though.

Any help is much appreciated!

[96CamrySport]

unless your car drives like crap or you burn a quart of oil every thousand miles, it sounds like your cat is dying. If you are under 8 years/80k miles, it's warranty. otherwise, expect to pay around $5-800, because you can't buy a used one. The egr system would need some actual diagnosis, and you didn't supply year/trim level/engine size for us to help.

[jackj]

Thanks for the reply. Sorry for the lack of details. It's an '01 LE, 4 cylinder. Hope that's all that is needed. It's got 101,000, so out of warranty. Curious why it couldn't be carbon buildup in the EGR or associated tubes, though.
Also, would the cat going cause the P0401 code? FWIW: the light's still out. I'm guessing that doesn't necessarily mean that the problem has resolved, though does it mean that it is temporarily (as would maybe happen if a piece of carbon got dislodged and cleared the passage?)
Also, if it is (temporarily) resolved, if I didn't clear the codes, I'm assuming they'd still be in there, even with the light no longing illuminated?
Thanks again!

[ShawnM]

The codes will still be stored in the ECM memory for some time after the light goes out. Typically those two codes are not related to eachother (one does not cause the other to set). I would say as long as its running fine just drive it till the light comes back on.

[jackj]

Thanks. Curious that two somewhat associated would trigger at the same time, though? Could it simply be a bad/dirty sensor(s)? Hmmm....

[96CamrySport]

unlikely to be a bad/dirty sensor, as it would be a downstream o2 sensor and it's signal would slow if bad/dirty and a fast switching signal (like upstream o2) will cause it to throw p0420. Mine came and went before staying for good. the light dissappeared because the computer hasn't seen the problem in 40 catalyst monitor runs. I think it stays in memory until cleared even if the light is off, regardless of time.

regarding the p0401, the gen4 is pretty similar to the gen3 mechanically and our egr valves have a tendency to gum up. Your theory works, but again, testing involves a vacuum gauge and ir thermometer.

sooner or later, you'll have to replace your cat. it may take a shop to diagnose your egr. I'm sorry I can't help more with it.

[jackj]

Thanks much! Fascinating, the machinations (no pun intended) involved in cars these days... Light is still off, so will continue to watch, and start saving my money for the replacement(s), I guess. Any thoughts on whether I'm best to go to the dealer, or a corner garage, that's reputable. I just wonder, as specific as cars are getting, if the average mechanic knows the specifics of each make/model, and can accurately diagnose what's truly going on.
Also, am I okay to keep driving until it comes on again? Could you explain what you mean by 40 catalyst monitor runs? And whether the problem is temporarily gone, and just not being noticed by the system? Thanks again!

[ShawnM]

The absolute best way to find a reputable shop (dealer or independant) is by word of mouth. Ask your friends and neighbors where they take their cars and who they trust. Naturally the places that have been around for a long time and have the latest diagnostic equipment will be the best bet. Dont assume the dealers have all the best diagnostic tools and experience. Yours cars computer system does a very good job of monitoring the various emission components and systems. If the check engine light is off and the ECM dosent see the error again after a certian number of drive cycles it will erase the code out of its own memory. Once it does that any shop you take it to will tell you that there is no problem with it because no current or history codes will display on the diagnostic scanner. The best thing to do is just drive it, and if the light comes back on take it to the shop at that time.

[jackj]

Thanks. I've been using a shop since I moved down here about 5 years ago, based on word-of-mouth, and my experience has been good in terms of trusting them, but they did mess up some diagnostics the last time, so my thought was that I trust them re: their honesty, just wasn't sure of their abilities. I don't know what diagnostic equipment they should have.
But my main thought was that what I've read on here is there are specific things that go wrong with generation 4, but not 3, or vice versa, that a dealer would be much more aware of. And that, while the reading may say the cat is bad, it's 1 of 4 possible problems, and they might be more aware of what the cause is, particularly, say, if two codes go at the same time, like with mine, that that would point in a specific direction that a "general" mechanic wouldn't be aware of. Thanks again for the help!