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Message: <blockquote data-quote="erlnmyr" data-source="post: 31548" data-attributes="member: 12426"><p>I just completed a head gasket and timing job on my 2017 equinox 2.4. It runs great on startup and during open loop and even in the start of closed loop but after about 15 to 20 minutes of idling well, some sort of trigger occurs and the upstream o2 sensor begins to read lean (.1-.2v) the downstream reads rich (.9) and the stft and ltft rise to upper 30s and stay there. This does not improve at 2500 rpm, we sprayed around the intake for vacuum leaks, i replaced the purge valve, and my current suspect is an exhaust leak before the upstream o2 sensor. The flange that joins the exhaust manifold to the cat has studs with very poor threads and I impacted the nuts on after hand starting due to degraded threads. Could this flange be sealing improperly and causing the upstream o2 to report false lean?</p></blockquote><p></p>