I have a friends Grand Caravan with the Pentastar 3.6 that recently threw a CEL and the code was a P0420. Right now, it's a history code, and when I hooked a scan tool up and pulled live O2 sensor data, the upstream sensors are switching, as they should and the downstream sensors are not fluctuating, as they should. When the cat is not operating right, it'll cause the downstream O2 to fluctuate as the PCM commands rich/lean because it's not converting the gasses as it should. My logging was only at idle, though. The FF for the P0420 was at highway speed or near it. Generally, a P0420 indicates the cat is going bad, however, from some research, I've read some blurbs about this engine having some other head issue that can cause this code, but with 13x,xxx on it, if that was the case, I'd suspect it would have come up sooner. Is there anything else particular to this engine/vehicle that I should be looking at that could also cause this code, or is it most likely as is usually the case, 'just' the cat going bad?