I have a check engine light code of P0135. "Upstream o2 sensor heating element circuit malfunction". It looks like I have 2 sensors upstream (directly underneath the manifold). How do I know which upstream to change and how do I determine #1 and #2 which I have seen mentioned in searching other threads? Is the #1 forward and the #2 aft?
Vehicle 2003 4.0L Wrangler