I have a 1999 TL, my CEL was on and the dealer said I needed a new CAT, which would cost $1400 installed. I had a local muffler shop install one for $400. After a couple of weeks, the CEL came on again with code P0420. The muffler shop told me that I needed a new rear oxygen sensor, which they installed (it came from Acura). The CEL light is back on with the same code P0420. Why does it continue to come on and what can I ask a mechanic to do to keep the light from coming back on? I am not handy with car repairs. Lastly, I am not due for a smog test in CA until 4/09. Can I continue to drive it with the light on without any problems?
Thank you for any assistance offered.

 

I'm guessing they put in an aftermarket convertor despite telling you it came from acura. Aftermarket convertors are seldom succesfull in keeping the light out. I just checked with my local acura dealer on pricing. There are 2 choices for a 99 tl based on vin#. The cheaper one lists at 776.87, 1030.00 for the other. My dealer cost on the "cheap" one is about 600.00. List prices are sometimes regionally adjusted, but this doesn't add up. Bottom line is they should make the light stay out or give your mioey back.

 

codes are not diagnosis
A good muffler shop would have tested the pressure inside the cat before replacement
They have to send a paper to the FEDERAL govt on every cat replaced- age-miles- reason- type of failure if known etc.
If the 99 has 2 O2 sensors, test the front one, and clear the codes with a code reader or disconnect the battery or clock backup fuse- that resets the master engine control and lets it learn that you fixed some very bad problems
drive with the light on and expect more expense before your smog test.
Note- when you reset the ecu- it can take a few weeks of driving before it will show readiness codes for smog test so get it fixed long before any test date- not the OP in this case but as a general warning to all

 

The 02 sensor was from Acura, but the cat was after-market. Acura quoted me $1200 for the cat and $200 to install.

 

It does have 2 02 sensors, so I will have the front one checked and erase the code and report back to you.

 

Code p0420 is set when the ecm sees too much activity at the rear o2 sensor. It's not the same as when a catalyst becomes restricted, so a backpressure test will show no problem. The real cure is almost always an oe convertor. I've heard of sensor simulators you can use to keep the light out as well as spark plug extensions to move the rear o2 sensor a little bit out of the exhaust stream. The one time I tried to get an extension, I couldn't find one to fit. If the fitting is in the middle of the convertor, the exhaust shop could try welding in a fitting at the rear of the convertor. I still think the exhaust shop owes you if they can't keep the code from recurring. I am assuming you went into the shop with a cel problem and not a driveability problem.

 

P0420 is for Catalyst efficiency below threshold. The aftermarket Cat that the aftermarket place installed is probably less efficient than the one that threw the code. Buck up and get the right parts to fix your car. its twice the price for a reason.

 

I took it to the muffler shop after the Acura tech told me that I needed a new Cat. The muffler shop did a scan and said I needed a ne Cat. For all I know, I may not have needed a new one. I did not have any driveability problems. Just the CEL on.

 

I have paid $400 for the Cat and $290 for installation of an Acura 02 sensor. I am not going to pay Acura $1400 for their Cat now.

 

Thanks for your analysis and recommendations.

 

Northeast Acura in Latham, NY quoted me $1557 yesterday (3/28/08) for a cat replacement on my '99 TL. When I balked, the service manager suddenly got generous and offered to split the cost 50/50 (after earlier insisting that he wouldn't waive the $100 diagnostic charge to pull a code I already had pulled).

 

Yes but my point was that you need to spend the 1400 to actually fix your car. Up until this point any money spent on the aftermarket stuff was a waste.

 

I just went through the same thing with my 99 TL. After having 4 aftermarket cats that all threw a CEL, I finally convinced the muffler shop to buy me a factory cat. I have had the factory cat for about a month now and I am code free! Factory cat FTMFW!!!