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Mercury Sable O2 sensor OR Catalytic Converter

So I took my car to a new mechanic today to have an O2 sensor and a leak looked at (Long story short, the leak is new, the problem with the O2 has been going on and off for a few months now. Fixed a brake line earlier and it took care of the original problem and now it’s back). He hooked up my car and then proceeded to tell me that I needed a new cat. converter and it would cost me $800 for the part alone because the part I need is a high end converter.The guy also told me that I could run my car without a problem even with the cat. converter not working properly. A few things don’t really add up for me. First, he didn’t put my car on a lift, he just hooked it up to the computer and barely looked at it. Is it possible to tell if it’s an O2 sensor problem or a converter problem just by a quick look? Second, I don’t believe that it’s entirely true that I can run my car without issue LONG TERM. What is the general consensus on that ruling?

I don’t care to have “parts thrown at problems”, so in your opinion should I get a second opinion or should I just grin and bear the big repair?

They can at least tell the cat is possibly a problem without much time or effort. They just connect their scan tool to the car’s obd II connector& look for cat diagnostic codes stored in the ECM memory. Your car must have posted one or more codes. Shops with more sophisticated equipment are able to inspect the sensor output directly from the pre-cat and post-cat O2 sensor, and from that confirm/deny the actual problem is the cat. If the only indication is a cat code, some more diagnostic work is a good idea before replacing the cat. Also worth doing an investigation for what caused the cat failure. Overly rich mixtures can cause that and if you just replace the cat, the new cat will be damaged by the same problem. $800 isn’t overly expensive for a cat, parts only. Re: running the engine w/defective cat. It depends on how it is defective. If it is passing the exhaust gasses freely, but not extracting the pollutants as much as it should, that won’t damage the engine. But if the cat is starting to clog up, or the ceramic matrix is disintegrating, that will cause performance and drivability problems and can in some cases cause major engine damage. There was a poster here recently who had a badly damaged engine that I think required complete engine replacement due to cat failure.

Thank you for your response! I think I understand better now about what the mechanic probably saw. I have a follow up with him in a few weeks to take a look at the leak so I will ask him some questions about what he saw when he looked at the car’s codes. And I will be sure to see if he can look into what caused the CAT failure. You seem very knowledgeable about how the CAT works and I’m glad you were able to explain it to me and ease my worries about driving my car around with a bad CAT. Thank you!

Best of luck.

Would be nice to know the model, year and mileage of the car and the error code(s).

I wouldn’t hastily agree that the mechanic did a good job identifying the cat as being bad. If he connected a life scan tool and monitored the voltage of the O2 sensors he may have come to a hasty decision. A faulty O2 sensor could throw a code that could be misinterpreted as a faulty cat. I would have expected him to measure the temperature at the inlet and outlet. A significant difference can be a good indicator of a bad cat.

If he only retrieved the fault code without a life scan I would get a second opinion.

Not sure what State you live in. If you must go through an emission test you will fail it.

Is the engine bugging down when driving? A clogged catalytic converter will do that.

When a mechanic connects a scanner with real-time monitoring to see if there’s a problem with the cat, here’s what they look for.



[quote=“jfinn002, post:1, topic:107397”]
So I took my car to a new mechanic today to have an O2 sensor and a leak looked at"

Are we looking at an exhaust leak because you say

If there is an exhaust leak…this must be addressed before considering any data from the cars computer to be valid.


He probably has P0420 and/or P0430

Personally, if I was doing any testing beyond using the scanner, dmm, smoke machine, visual inspection, etc. . . . I’d be more tempted to put the car on the dyno and check out the results from the 5 gas analyzer, for diagnostic purposes