Am I in denial?

I purchased a 2004 auto with high mileage. 10 days after I purchased it, the Service Engine Soon light came on. Code: P0430. (Catalyst system efficiency below threshold (Bank 2) I reset the SES light. Needless to say, the light came back on again about 6 weeks later. Took it back to the dealer where purchased. The dealer’s mechanic says "sorry, but you need two catalytic converters and possibly up to 4 new 02 sensors to boot. I asked if he tested the O2 sensor downstream from the converter in question (Bank 2). He said there’s really no reason to. He knows the cat is dead. And if one is bad, the other converter can’t be far behind. Perhaps I’m in denial but couldn’t a faulty O2 sensor cause an incorrect reading? Or at least be ruled out as a possible problem? Or is it highly unlikely the sensor is bad because the SES light came back on again? Thanks for your input.

A competent mechanic should be able to figure out if this is simply a case of bad O2 sensor(s), or if it is a case of cat converter failure. The possibility of the cat(s) being bad is so much lower than the probability of just a bad O2 sensor that I would suggest you take the car elsewhere for diagnosis and repair.

Well no. There’s no reason for him to test anything. His objective is to take in as much profitable business as possible and to be so thorough that there will be no comebacks. It would seem worthwhile for you to pay a qualified mechanic to look at the live data from your vehicles computer and determine more precisely what repairs are needed. I feel sure that if the SES light had illuminated and that code appeared during your test drive of the vehicle a great deal less would have been done at their expense to sell the car than they recommend now at your expense.

Indeed, like Rod Knox said & fitting with VDC’s comments - this is the response of a place that operates with the primary objective of selling you stuff.

This mechanic is full of crap. First, you can’t “know” the cat is bad just from a code. Second, the notion that if one cat goes the other isn’t far behind is absurd. This would work for things like wheel bearings which are wear & tear items. Cats are just not like that.

Of course, none of this means that you don’t need a catalytic converter. Its just that you don’t know yet and yes, O2 sensor problems can cause this. So can things like exhaust leaks.

Just FYI the only thing this code means is that the readings from the front (upstream) O2 sensor and the rear (downstream) O2 sensor are too similar. The upstream sensor is generally very active doing a lot of bouncing up & down in its voltage range. The rear sensor should be much much less active because the cat is cleaning up the exhaust. When the pre and post cat readings are too much alike the code is set.

How many miles are on it?


I had the exact problem on a Crown Vic and replacing the downstream oxygen sensors cured it.

The sensors have a design life of 100K miles. I would change them before I would replace the converters…

The mechanic said if it were the O2 sensor then a code for the faulty O2 sensor would have shown up instead. Therefore, it can only be a bad cat.

If you are satisfied with the dealership’s mechanic’s diagnosis, why are you here?

As Rod Knox said…

So you can believe us or the mechanic, I suppose. But the mechanic doesn’t know what he is talking about. And/or he’s just trying to sell you a whole bunch of work that you only might need.

Oxygen sensors can slowly degrade and provide an incorrect signal ( a voltage actually) without generating a trouble code of their own…The computer condemns the converter but the signal is coming from the 02 sensor…

Bad 02 sensor can be the cause of catalyst efficiency related codes. I’d definitely try the 02 sensor first. This mechanic is trying to sell you possibly a few thousand dollars worth of converters and 02 sensors depending on the make and model of your car, where a $100. 02 sensor might take care of the problem.

In regards to Rod Knox comment: I am not convinced the proper steps were taken by this mechanic to access my problem. I am only trying to represent the mechanic’s diagnosis and opinions accurately in hopes of gaining ammunition from you folks should I need to discuss this matter further with the dealer or mechanic. Not confront him, but merely ask questions that will lead me to the proper course of action. I appreciate everyone’s advice. Thank you.

Well, my opinions on Car Talk are usually consicered crap. I an just an old car buff who has done my own repairs for 50 years. The last 10 years I have had to rely on more qualified mechanics, because I don’t like to work under a car any more and the new technical stuff is over my head. I say all this to say that I believe the boys (above) are right on. I always look for the most inexpensive cure first.

I had the same code. It was the cat, but you have to check the O2 function first, and check for a cracked manifold before replacing the cat. There also is no reason in the world to replace both of them if only one is bad.

For whatever my opinion is worth, it seems that the dealership’s mechanic made only a perfunctory code scan with no effort to follow up with specific testing of each component and the system as a whole. If an O2 sensor proved to be faulty while the catalyst was questionable it would be sensible to replace the sensor and inform you of the possibility that the catalyst may be failing. We here are unaware of the dealership and the expertise and ethics of the shop but for me, based solely on the limited information you have posted I would recommend not returning to that shop. But I am “shooting from the hip” as they say.

So this guy sold you a car, and then a month later wants you to believe that now you need two cats and four oxygen sensors? And that when he sold you the car everything was just perfect? And so, for a couple of thousand dollars he says the car will be fine? And you are feeling, shall we say, a little uncertain about if he might be trying to rip you off?

Don’t be uncertain. He is.

Go somewhere else and get a second opinion. And then tell the state department of consumer protection or whatever regulates dealers that this guy is a bad apple.

I agree. A minute ago, I posted on a similar low efficiency cat question. When I had a problem, I Googled, and almost everyone, mechanics, etc., said most such failures are the sensors. Period.

But, do not go back to that mechanic. He has proven he is not working for you.

Check out mechanic files on this URL for a competent mechanic in your area, and take your car there.