09/06/2026 6:12 am
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Automotive Q&A
P0420 returned after an exhaust repair and the rear O2 still follows the front
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A 2010 Corolla had a leaking front pipe flange repaired, but P0420 returned after two monitor runs. Fuel trims stay within 5%, the upstream air-fuel sensor responds normally, and the rear oxygen sensor now switches more than expected at a steady cruise. No rattle is heard from the converter.
Before calling the catalyst inefficient, how should I confirm that the exhaust is actually sealed and that the rear signal is not being distorted by another fault?
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10/06/2026 7:53 am
Repeat a low-pressure smoke test of the exhaust while it is cool and inspect both sides of the catalyst, sensor threads and the repaired flange. Even a small upstream leak can add oxygen and change the rear trace. Verify that the rear sensor heater, bias voltage and response are normal, and compare front and rear activity only after closed loop is stable at the enable conditions from the freeze frame.
If the exhaust is sealed, trims are controlled and the rear sensor continues to mirror oxygen-storage events, catalyst efficiency is the remaining conclusion. Do not use temperature alone as a pass/fail test.
Topic starter
11/06/2026 9:15 am
A thin smoke trail appeared at the upper side of the repaired flange where the gasket had not seated. Resealing it reduced rear-sensor activity substantially. The catalyst monitor completed twice without P0420, so the converter was not replaced.