So, I thought I'd share the little saga I've had with my FJC of late. Cliffnotes: I've been throwing P0420/P0430 (catalytic inefficiency right/left bank) codes since earlier this spring and now have to replace the exhaust manifolds. History: 06/06 build date, purchased 07/06. 122,000 on the ODO. Daily driver and weekend wheeler (more like bi-annually these days). Heavily armored, meticulously maintained. Stock drivetrain except 4.56 gears and 35" KM2's. Never had a CEL come on before. Only overheated once or twice in the first year of ownership when radiator was blocked with mud (highest temp registered was 220). Stock air intake and exhaust - with only a minor modification to run the exhaust pipe behind the t-case instead of under it. Symptoms: I started throwing these codes every 500-600 miles earlier this spring. I asked around a bit and everyone said, "oh its just your O2 sensors behind the primary catalytic converters - a common Toyota problem. Just replace those and your CEL will go away." Diagnosis: When I replaced the O2 sensors ($100 each) the codes did go away for about six weeks. Ever since then, the frequency of the CEL coming on has increased. I've been scanning and resetting the codes with my ScanGaugeII. Usually it would only set off the left bank or the right bank at any time the CEL came on. I think once it registered both left and right at the same time. I've got to pass emissions next month. Now they come on every 30 miles or so which makes it impossible to pass the "readiness" test at the emissions center. The readiness cycle of your ECU is around 50 miles or so of mixed highway/city driving. Every time I delete the codes with the ScanGaugeII, it resets the readiness cycle of the ECU. If the ECU is not "ready", you fail the emissions test by default - kind of like a DQ in racing speak. The next test was to measure the temperature of the exhaust going in and coming out of the primary cat - done with an infrared/laser temp gauge pointed at the exhaust pipe before the cat and after the cat. If the cat is working properly it should be hotter (delta of 100* or so) from catalyzing the gases on the back side. Mine was reading an average of 650* in and 650* out. So that definitively narrows it down to the primary cats not working at all. Anatomy and Cure: The primary cats on the FJC are integrated into the exhaust manifold. No one makes a direct bolt on replacement other than Toyota. If I go aftermarket, then I'm skyrocketing my costs with JBA headers and Magnaflow cats. I ordered new manifolds and gaskets from Sam at Lowe Toyota on Monday and hope to have it all 'd out this Saturday. I'll update post surgery. If you're lucky, I'll even get a few pics of the procedure.