Hello. I have the Po171 code on on my 03 gt saying bank 1 is running too lean. I just changed my o2 sensor on that side and did the lowers over the summer. Why is it only one side running lean? I cleaned my throttle body and iac valve earlier today but still running lean. Could i have a leak in my header somewhere? or could it be the fact that the bing thing tha the sensor screws into had popped out a while ago and i tack welded it back in and thought i did a good job possibly? i really appreciate any help

 

Check for any exhaust leaks before the Upstream 02 sensors and where the headers meet the engine. That causes excess oxygen to seep into the system and the 02 sensor will pick that up thinking the engine is running too lean, instead of it actually being just an exhaust leak.

Bank 1 is the passenger side so check for any sort of vaccum leaks there too. Perhaps the Intake Manifold is leaking on that side. Have you done any replacement to that recently? Check PCV Valves on the engine for that side. It's a possibility that if you replaced your headers in the past they weren't bolted on right. There is a lot to consider here