Thanks jb for your response.
Guys, I'm getting a little desperate here. I've been monitoring the O2 sensor voltages before and after the precat for both banks 1 & 2.
First off, while ideling, both banks look fine. Before cat is sinusoidal and after cat is fairly flat at around 0.8v.
While driving, the after cat voltages are fairly jumpy with bank 2 being a little worse. However, the before voltages aren't consistently up & down and seem to skip a beat quite often.
I am getting desperate because I don't want to spend 6 hours on my back changing out the precats just to find out the problem is still there. (I am a pretty accomplished DIY mechanic but having to wear reading glasses most of the time has taken a lot of the fun out of it.)
To recap the problem, the cat efficiency monitor is stuck in Fail state and won't switch to Pass even after ~300 miles of driving. The other 4 monitors are in Pass state and I haven't got another CEL since I turned off the last one and tried to smog the car 9 days ago. It's a 2002 Boxster base with 96 Kmiles.
Please respond if you can help me diagnose the problem with more accuracy.
Thanks again.
PS, If I let the car idle for a minute or two, the period on the sine wave becomes as tight (short) as we see in most illustrations. However, the period is much longer most other times.