Hey all - I'm new here, but have been riding a CRF250L for about a year.
I broke the O2 sensor on my '17 model while switching to an FMF setup last weekend. It seized up about halfway out of the stock header pipe and I ended up snapping the sensor in half at the threads. No local dealerships had an O2 sensor in stock, so I ended up using an M12-1.25 oil drain plug (for Nissan, Autozone part # 653056 worked perfectly) as a stopgap solution so I could go trail riding with my son the following day.
I added an EJK at the same time, adjusted per this post and the bike ran very well both on the street and on the ORV trails we rode. I can tell it is definitely running richer now, as the fan didn't kick on nearly as much as before while riding in the woods and there is a slight bit of soot accumulating in the tailpipe. It did occasionally pop and crackle a bit when coming off a handful of throttle.
Although I'll probably end up buying a new O2 sensor eventually, I'm actually very pleased with how the bike runs as-is. Other than improved fuel economy and eliminating the check-engine light, are there other advantages/disadvantages to running with an O2 sensor? Is there a way to avoid a persistent check engine light on 2017+ models when running without an O2 sensor?