P0430 — Catalyst System Efficiency Below Threshold (Bank 2)
P0430 is the bank 2 twin of P0420 — the same fault, on the cylinder bank that does not contain cylinder 1. It is usually safe to drive short-term but will fail an emissions test.
The vehicle will drive normally. An underlying fault left unrepaired will destroy the replacement converter as surely as it destroyed the first. This code will cause an emissions test failure.
What P0430 means
On a V-configuration or flat engine the cylinders are split into two banks, each with its own exhaust path, converter and oxygen sensors. Bank 2 is the bank that does not contain cylinder 1. The module grades each converter separately by comparing the sensors before and after it, so P0430 means the same test failed on the other side. Everything about the diagnosis matches P0420 — including that a slow rear sensor imitates a spent converter closely enough to cause the most expensive misdiagnosis in the code.
| System | Emission control |
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Symptoms you may notice
What causes P0430
Bank 2 catalytic converter has lost efficiency
The substrate is contaminated or broken down and no longer stores and releases oxygen. Most likely on high-mileage vehicles, and more likely still if a misfire or rich condition went unrepaired.
Bank 2 downstream oxygen sensor is slow
An ageing rear sensor produces a trace that resembles converter failure. Replacing a converter on this fault is the single most expensive mistake this code invites.
Common misdiagnosis
Graph both bank 2 sensors before quoting a converter, and compare against bank 1 — it gives you a healthy reference on the same vehicle.
Exhaust leak on the bank 2 pipe
A leak upstream of the rear sensor draws ambient air into the stream between pulses, skewing the reading lean and imitating converter failure.
Misfire or rich condition on bank 2
Raw fuel entering the converter overheats and destroys the substrate. If P0430 arrives with misfire or fuel trim codes, repair those first — the converter is the casualty, not the cause.
How to diagnose it
Identify which bank is bank 2 on this engine
Bank 2 is the bank that does not contain cylinder 1. Confirm against the engine layout before touching anything, because working on the wrong side wastes the entire diagnosis.
Read every stored code first
Scan for all codes. Misfire, fuel trim or oxygen sensor codes stored alongside P0430 identify the real fault, and the converter is the symptom.
Graph both bank 2 oxygen sensors, and compare against bank 1
At a steady 2500 rpm and full operating temperature, the upstream sensor should swing rapidly while the downstream one stays nearly flat. Bank 1 gives you a known-good comparison on the same engine.
Inspect the bank 2 exhaust for leaks
With the engine cold and the vehicle safely supported, inspect the manifold, flex section and every joint ahead of the rear sensor for soot trails and cracks.
Safety
Never work under a vehicle supported only by a jack, and let the exhaust cool completely first.
Related codes
Common questions
What is the difference between P0420 and P0430?
Only which bank. P0420 is the bank containing cylinder 1, P0430 is the other one. The fault, the causes and the diagnosis are identical.
Can a four-cylinder engine set P0430?
It should not. A single-bank engine has no bank 2, so P0430 on an inline four usually points at a wiring or module fault rather than a converter.
Do I have to replace both converters?
No. The banks are graded separately, and only the one that failed the test needs attention. Replacing both because one failed is a common and expensive upsell.
Last verified 2026-07-27 · Sources: SAE J2012 — Diagnostic Trouble Code Definitions and Failure Type Byte (SAE International)
This information is for general guidance and is not a substitute for professional diagnosis. Cost figures are estimates, not quotes.