I am trying to help you limit your repair bill at this point. Because there are so many other things (transmission, especially) that are likely to fail on this 17 year old vehicle within the near future, and because its book value is so low, that cat replacement money would be better off in a savings account for purchasing the next vehicle.
The way that you spend your money is your decision, but if you look at the probable remaining life in this vehicle, I think that conserving monetary resources at this point would be a wise thing to do.
Thanks, I do appreciate your comments.
Just fyi, I put a new transmission in at about 88,000 miles. I don’t know how long a lifespan that might have. I had a tuneup a few weeks ago. Does that usually check spark plugs?
The truth almost parts for vehicles are made by “aftermarket” auto suppliers and branded as OEM. Quality ranges from junk to better than OEM parts.
I would not worry about an aftermarket part from reputable supplier or auto parts maker.
Two things have major effect of the production of NOx in the combustion chamber: exhaust gas flow into the intake air, and on into the combustion chamber; and ignition timing.
Ensure that the EGR valve is flowing exhaust gases into the intake air; and, set the ignition timing as close to retard as you can. The engine will have to continue to run smoothly; so, not too much retard.
The catalytic converter can be poisoned by an engine burning oil, or antifreeze, or too much unburned fuel passing through the engine.
The spark plugs reflect these conditions. Here are pictures of some spark plugs. Your mechanic should have “read” the old spark plugs. http://www.autolite.com/pdf/PlugTips.pdf
Kit’s suggestion about checking the EGR valve is a good one.
However, in a 17 year old vehicle with 150,000 miles I’d want to look closely at the cat converter. The function of the converter’s first stage is to seperate the NOx into nitrogen and oxygen, its two components, in order to reduce NOx output and to provide free oxygen for the rest of the converter to perform eom other emission-reducing chemistry. It does this by passing the exhaust through a platinum-palladium coated honeycomb. The platinum-paladium seperates the NOx molecules that come in contact with it.
If the coating on the honeycomb is contaminated, it will not perform this function effectively and the NOx readings will be higher than they should be. This often happens because as the engine wears and begins to burn a bit of oil the oil residue coats the platinum-palladium. On a car this age with this milage it would not be unusual.
The mechanic may have told you that he cannnot guerantee it’ll pass because he has little way of checking the performance of the converter itself. On newer cars there is an oxygen sensor before the cat converter and one after, and the two signals can be compared, but on your car there’s none after the converter. A comparison cannnot be performed.
Back to Kit’s comment. Excessive NOx, along with preignition, can be produced when the cylinders get too hot. The EGR valve allows recirculation of a bit of inert exhaust gas when the engine is placed under load to displace some oxygen and prevent the cylinders from getting too hot. That reduces the formation of NOx molecules. Too much oxygen can also cause a temperature rise, and that’s where his lean operation comment comes into play. Retarding the ignition can reduce heat too, but that becomes a performance tradeoff.
In short, on this vehicle with its age and miles I’d be inclined to try an aftermarket direct-fit converter. It’s old enough for the honeycomb to be coated.