Yesterday out of nowhere my SES light came on - this has happened before and I have had to replace sensors. Then it started blinking and was "puttering" and shaking as I drove it. I had it towed to my car care center and just wonder what input anyone has. Definitely seems more like a sensor that needs to be replaced and it has never done the shaking thing before.

I like the car but it surely has been expensive to keep replacing sensors and I had to have the radiator replaced in July - it is only 4 years old! Aren't these supposed to be reliable?

A SES blinking means that catastrophic cat failure is occuring. It means the engine is missing to a point the cat will be destroyed if you keep driving it. You simply have a misfire. I am guessing its is one or more if the coils dropping out. The codes pulled will likely indicate the failing coil. You should see a p1320 and a p030x [x being the cyl number that was missing.] replace that coil(s) and all should be well, and hope there is no p0420 or p0430 stored as that would indicate cat failure which by the way is covered under 8/80 [8 yrs or 80,000 mile warranty(which ever comes first)]

My '01 Pathfinder had many DTC codes which have been fixed with with replacements for the two primary Oxygen censors, an Intake Valve Timing Censor, the cylinder 2 coil. However, after everything was done, the SES light came on again with two code P0420 and P0430. Look like the cat has failed. What can I do to know for sure what is failing?

The prior failures were likely the cause of the cat failures. It is likely it only damaged it enough to be above the failure margin. The rest of the damage likely occured during normal wear. However before you slap the cats on you should have the A/F alpha checked by an experianced nissan tech with a consult to determine if you are running on the lean side. Would be a good idea to get the MAF tested as well. Say your A/F alpha looks good at 2000 rpm [nissans test point] But when your accelerating onto the freeway or at a different steady rpm interval your A/F alpha shoots through the roof (very lean) you cat is being damaged. This could be due to a bad MAF not properly registering the intake air.