P0420 Fault on 1998 Toyota Tacoma (160K Miles) - Causes and Diagnosis

I recently received a P042-0 OBDII diagnostic code on my 1998 Toyota Tacoma, which has accumulated 160,000 miles. Before this issue arose, I performed routine maintenance including replacing the spark plugs, air filter, and adding a fuel injector cleaner to prepare for winter and improve fuel efficiency. Prior to these changes, the truck exhibited rough idling and difficulty starting on damp mornings. Additionally, I noticed an intermittent 'rotten egg' odor during both city and highway driving. Fuel economy improved slightly after the first tank of gas, but the check engine light activated midway through the second tank while driving on the highway. I'm wondering whether the fuel injector additive I used could be responsible for triggering the P0420 code, or if there's another underlying cause. It's worth noting that both upstream and downstream oxygen sensors were replaced a year ago. Any insights or troubleshooting steps would be greatly appreciated.