Calling on 986 Forum Master Mechanics

[Jager]

Oh no... I just got a P0420 error code. Over 284,000 miles on CAT bank 1.

[JFP in PA]

How about we start with what codes is the car throwing, and what voltages are seeing for the O2 sensors ahead of the cats and behind them?

[seventythree]

For about 6 months I had been getting P0430. Only once I got a P0420.
But for the past 10 days and 350 miles, since cat efficiency monitor has been in fail state, no codes whatsoever.
When I start the car, all four voltages fluctuate between .05 and .8 volts.
Once warmed up, while driving on the freeway at around 70mph, both voltages ahead of cat fluctuate between .1 and .75 volts; voltages after fluctuate from .5 to .75 volts with bank 2 fluctuating more, and more often than bank 1.
While decelerating under engine braking all four drop to near zero.
After a long drive, if I let the engine idle for 2-3 minutes, voltages ahead fluctuate between .05 & .8 volts; voltages after from .6 to .75 with bank 2 fluctuating more.

I hope this is sheds more light on the problem.

[JFP in PA]

Originally Posted by seventythree

For about 6 months I had been getting P0430. Only once I got a P0420.
But for the past 10 days and 350 miles, since cat efficiency monitor has been in fail state, no codes whatsoever.
When I start the car, all four voltages fluctuate between .05 and .8 volts.
Once warmed up, while driving on the freeway at around 70mph, both voltages ahead of cat fluctuate between .1 and .75 volts; voltages after fluctuate from .5 to .75 volts with bank 2 fluctuating more, and more often than bank 1.
While decelerating under engine braking all four drop to near zero.
After a long drive, if I let the engine idle for 2-3 minutes, voltages ahead fluctuate between .05 & .8 volts; voltages after from .6 to .75 with bank 2 fluctuating more.

I hope this is sheds more light on the problem.

Your sensors are fine, you have a cat issue.

[Swhitcomb]

Got an 0430 on my 2002 Boxster yesterday. Cleared it and it hasn't returned.

Pulled this from Torque

http://youtu.be/oKFTjd00a6Y

Cat bad or O2 sensor?

[Gelbster]

People with a suspect cat are reluctant to spend the money on an expensive replacement if they are unsure of both the diagnosis and result.But with a looming Smog Test there is only one cheap diagnostic test I can think of .It is not a permanent fix and it is illegal to drive with the test parts in place. It is just to use long enough to monitor the voltage of all 4 O2 sensors.
Use SHORT anti-foulers or extenders on the rear O 2 sensor of the failing(?) cat . On Bank 2 there is space.Bank 1 requires more creativity.
You can also try cleaning the cat. Neither of these are smart fixes imho but they may ' get you by' - but just to test. Be aware the extender may cause a lean code !
The extender will cause the rear O2 sensor voltage to 'flat-line' and look like the cat is working. This is your clue to replace the cat - assuming all the other suspects have been eliminated/replaced with correct ,new parts - coils,O2 sensors,plugs,vac leaks all other related codes.

[Timco]

Um, you just answered a thread from over a year ago....

Better late than never.

[PaulE]

FWIW I have the Fabspeed Sport headers with catalysts, Circuit Werks mid pipes without catalysts, just small silencers in their place, and the stock U pipes and muffler on my 2003 S which was rebuilt as a 3.6 and has a custom Softronic tune. I have the stock O2 sensors, not those special sensors designed to trick the ECU. My car passed the NJ State inspection at a State DMV inspection center, where the only things done are to see that your check engine light is off and hook up their OBD system to your car and make sure it isn't throwing any codes.