What are the symptoms of a bad catalytic converter?

In most TN counties, we have no emissions testing, so the Cats I have had which have failed had to be really bad to show up in terms of performance of the car -- but that is exactly what happened. When a Cat completely dies, the honeycomb structures inside it collapses. This causes a couple of things: 1) with all that stuff loose and laying around internally, you can hear it rattling as you drive. To check this, just grab ahold of the exhaust and give it a shake. You'll hear it. 2) The loose material can actually block the flow of your exhaust which significantly effects the performance of your vehicle. If that happens, you will loose about 80 of you 114 h.p. (or is it 116??). Anyway, there is no mistaking it at that point.

If I recall correctly, all 3-way cats have the reduction catalyst first and then the oxidation catalyst. NOx is controlled in the reduction end. The quick answer to your question is: The only way to tell for sure if the NOx reduction end of the catalyst is working is an emissions test.

A reduction reaction means that it separates the oxygen from the nitrogen, converting NO and NO2 gas molecules back to N2 and O2. The oxygen that is freed up in the reduction reaction is then available for use by the second half of the cat, the oxidation end, to 'burn' the excess CO and hydrocarbons to make CO2 and water.

The reduction catalyst can fail because it is poisioned by contaminants in your fuel or engine sealants. It can also fail because it is plugged with ash from oil consumption or plugged/coated with oil sludge from bad exhaust valve guide seals.

A good reduction catalyst can fail to reduce NOx if there is too much oxygen coming in to it. It cannot promote a reduction reaction in an excess oxygen environment. But the things that would cause too much oxygen to hit the front of your catalyst would have caused other problems on your emissions test.

My newly aquired 1987 Volvo 240DL just failed NJ inspection because of emmissions. Catalitic converter needed to be replaced. One indication that Cat is bad is if NoX (Nitrous Oxide) is too high. Mine was nearly 4X times the allowable amount. Funny thing about Cats is they could look fine(mine looked really fine)visually and yet be bad. Not a cheap repair. With installation replacing mine cost $250. Hope this helps.

nt