Catalyst System Efficiency Below Threshold (Bank 2)
Same as P0420 but on bank 2 (the other side of a V-engine).
What it means (plain English)
Identical story to P0420 — the PCM is watching the bank 2 oxygen sensors and noticing the downstream sensor is now mirroring the upstream switching activity. That means the converter on that bank is no longer cleaning the exhaust. On a V6 or V8, bank 2 is the side that doesn't have cylinder #1.
What the computer is actually seeing
B2S2 voltage is tracking too closely with B2S1. Catalyst monitor on bank 2 flags efficiency below threshold.
What a healthy reading looks like
B2S2 should sit steady around 0.7 V in closed loop. B2S1 switching 0.1–0.9 V at 1–2 Hz. Downstream flat, upstream busy.
Guided diagnostic — the DiagCoach way
Don't just throw parts at it. Walk through these in order — each step tells you whether to keep going or stop and fix what you found.
- 1Check for any bank-2 specific codes — injector, misfire, fuel trim.
- 2Compare bank 1 vs bank 2 long term fuel trims. Big split points at one bank's fueling.
- 3Graph B2S1 and B2S2 at 2500 RPM. Downstream should be flat near 0.7 V.
- 4Inspect bank 2 exhaust manifold and pipe up to the rear sensor for leaks.
- 5Temp-test the bank 2 cat (outlet ~100°F hotter than inlet when working).
Common causes
- Worn cat on bank 2
- Lazy B2S2 oxygen sensor
- Exhaust leak on bank 2
- Rich condition specific to bank 2 (leaking injector, intake leak)
- Misfire on a bank 2 cylinder
Typical repair cost
$200–$2,500. Often paired with P0420 if both cats are old.
Related codes
Frequently asked questions
Why only one bank?
Each bank has its own exhaust path, cats, and O2 sensors. One can fail while the other is fine — especially if a fuel injector or coil is weak on just that bank.
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Let DiagCoach walk you through it live with your specific symptoms, vehicle, and what you've already checked.
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