P0420 bank 1. Customer says it's a comeback.
I see the history —
cat bank 1 was replaced at this shop 8 months ago at 98,104 miles for this exact code. Current odometer 112,400.
Mode 07 shows a pending
P0302 — cylinder 2 misfire — not mature enough to set a confirmed DTC yet, but it's recurring.
Intermittent misfires are the #1 killer of catalytic converters. Raw fuel dumps onto the hot substrate, combusts there, melts the washcoat. The original cat was likely killed by cyl 2 misfiring. If you replace the cat again without finding the misfire, you'll be back here in six months.
Before touching the cat: verify cyl 2. Per MOTOR diagnostic tree — scope primary ignition or do a coil swap. B1S2 is currently
0.82v steady — elevated but not yet mimicking upstream, meaning the
new cat is still within tolerance if we catch this now.
◉ Cited: MOTOR Diagnostic Tree P0420 · CFP Repair History
Coil primary looks weak on cyl 2. Swapped 2 with 4 — pending moved to P0304.
That's your root cause. Coil 2 is the failure. The 3.5L EcoBoost has a well-documented coil primary winding breakdown — there's a TSB covering it on 2017–2020 F-150s.
Replace coil 2, clear codes, drive 100 miles to complete a cat monitor cycle. Watch B1S2 — if it returns to 0.6–0.7v steady and LTFT stays within ±5%, the new cat survives.
Document the coil replacement on this ticket. If this cat ever comes up for warranty consideration, the history shows you caught and corrected the root cause on the comeback — that's the protection.
