My 2007 LR3 with 135,000 miles is throwing P420 and related codes. I have been told by a Land Rover mechanic that it's not the O2 sensors as I initially suspected, but the catalytic converters are in fact bad, and will need to be replaced to get the check engine light to stop coming on. Was quoted $3,600 to swap out both cats. The job does not appear to involve welding. Has anyone done an LR3 catalytic converter replacement themselves? If so, how difficult was it? How long did it take and what sort of challenges were involved?