Fit NOx Catalyst Aging

The symptoms you describe suggest an exhaust leak is a highly probable root cause. Error codes pertaining to the NOx catalyst are frequently triggered by even minor exhaust leaks in the immediate vicinity. Before considering a catalyst replacement, a thorough inspection of the exhaust system is warranted. Pay particular attention to joints, welds, and any areas exhibiting corrosion. Has there been any recent work performed on the exhaust system?

No recent work on the exhaust system, thankfully. Where exactly should I be sniffing around for this leak? Any specific areas on the Fit's exhaust system that are notorious for cracking or rusting?

While I cannot direct you to a specific location without a physical inspection, I can offer a generalized assessment. On vehicles with similar exhaust configurations, the flex pipe and the joints connecting the catalyst to the exhaust manifold and the downstream exhaust components are common points of failure. Given the nature of the diagnostic and the potential for further system damage, I would recommend consulting with a qualified automotive technician. Attempting repairs without proper tools or expertise could exacerbate the problem.

Took it to the workshop, and you were spot on! Turns out there was a small crack in the exhaust manifold. They welded it up, cleared the codes, and all seems good now. Total cost: 265€. Much cheaper than a new NOx catalyst! Appreciate the cautious advice!