I have an ongoing issue with an 8FGCU25 and a check engine light - E 01-5. I have replaced the regulator ( for a rolling idle) and the O2 sensor. About 20 minutes into use the code reappears. The exhaust smells like rotten eggs as if a vacuum leak, but none found. On the screen I can see the O2 voltage fluctuate ( 21 - 38) and 0-1 switching. I have cleaned the carb, but not any of the valves as in prior posts. Any ideas?