HouseCallAuto 11/12/2010The B1S1 Oxygen (O2) sensor is the passenger side upstream O2 sensor. Replace it and probability of code P0135 not returning is greater than 75%. P0300 is a random misfire. In the absence of other codes besides P0135, it can have multiple causes however the most common cause is a faulty air flow meter. Here is the air flow meter for $110 and here is the O2 sensor for $63. So total of $173 in parts - free shipping. The air flow meter is a piece of cake to change and the O2 sensor is a little harder than a walk in the park. Try it yourself as you have a high probability of solving the problem with both codes.