The 2003 Forester had this code before engine swap from 2.5 to 2.2 and still has it. p0030. I cleared it once and it came back so now for the track down to see if it is the O2 sensor or if it is the relay/ bad ground in ecu. 

 So first question is where is the O2 sensor relay?

2. Which wires do I check with my ohm meter to test the heater circuit.

3 What is a good replacement if it is the O2 sensor?

I know the connectors are good and tight + clean.

Edited June 15, 201510 yr by desertsubaru

Check the heater for resistance. The heater wires on the sensor are the two that are the same color. Check the corresponding wires in the harness for 12v with the key On. Whichever that is, check the other wire for continuity to ground.

 

I don't have wire diagrams for the 03 forester. If you look around here there's a link to a site with factory service manuals for almost every year and model.