A fouled O2 sensor can/will show a fault as well. In twin sensor setups, the forward sensor usually sees hot enough exhaust gas to stay clean, and the rear one usually sees enough heat to stay clean since it's immediately after the cat, or inside it. Since you don't have a cat anymore, it is possible that the engine is running rich, which makes the exhaust colder than it ordinarily would be, and could be cold enough to allow carbon fouling by the time the exhaust reaches the second O2 sensor.

First things first, let's stop with the "what ifs", grab a wrench and take a look at your O2 sensors. They should be a whiteish tan color if they're doing fine, otherwise, there's a strong chance it's fouled and causing your fault. If that is the case, try one of the three methods I offered earlier to prevent the second sensor from fouling, which will also take that CEL off your dash.