My 2004 TL has codes P0135(O2 Sensor Heater Circuit Malfunction (Bank 1 Sensor 1)
) and P0155(O2 Sensor Heater Circuit Malfunction (Bank 2 Sensor 1) ). I understand its 4 sensors total so Im assuming I only need to replace 2. But which 2 are these? front,rear,downstream,upstream?? im clueless and I just want to make sure I order the right one.

 

Those two senors closest to the top of your engine, they are front and rear upstream. Upstream in other words is before the cat, Downsteam being after.