I would think so, but at the same time, theoreticially the o2 sensor and the ecu should try and keep the mixture correct regardless of fuel pressure, since the ecu has no idea what the fuel pressure is. If the ecu sees a lean mixture it would increase the duration of the injection, so even with low fuel pressure it should work, but i'm certainly not an expert on the system so hopefully someone with more knowledge can chime in. As a thought, how many miles are one your o2 sensor? and if its aftermarket is it the kinda you crimp on to the old connector or direct oem style?