P0430: Catalyst Efficiency (Bank 2)
Table of contents
Key Takeaway
P0430 is P0420's twin — same issue, Bank 2. Here's what you need to know.
P0430 is the catalyst efficiency code for Bank 2 (the side of the engine opposite cylinder 1 on V-engines). Diagnosis and causes are identical to P0420 — a degraded catalyst, upstream misfire damage, a failing rear O2 sensor, or an exhaust leak. Inline engines do not have a Bank 2 and will never see P0430. STEER reads both codes and identifies which bank is affected.
What Is Bank 2?
| Engine Type | Bank 1 | Bank 2 |
|---|---|---|
| Inline 4/6 | Only one bank | N/A (will not get P0430) |
| V6/V8 | Side with cylinder 1 | Opposite side |
P0430 vs P0420
| Code | Bank | Same diagnosis? |
|---|---|---|
| P0420 | Bank 1 | Yes |
| P0430 | Bank 2 | Yes — same steps, different side |
| Both together | Both cats failing | Likely misfires damaging both |
Having Both P0420 + P0430
If both codes appear, the root cause is likely upstream of the catalytic converters — misfires, running rich, or ignition problems that damaged both converters. See our [OBD-II codes pillar](/codes/) for the broader catalyst diagnostic flow.
STEER tells you which bank, which sensor
On a V engine, knowing whether the fault is Bank 1 or Bank 2 saves time at the shop. The [STEER OBD-II scanner](/obd2-scanner/) reads the full code (P0430) plus the live O2 sensor switching pattern, so you can see whether the post-cat sensor is genuinely flatlined (real catalyst issue) or just sluggish (sensor going bad). Pair with the [AI Mechanic](/ai-mechanic/) for plain-English next steps.
Frequently Asked Questions
What is the difference between P0420 and P0430?
Identical fault, different physical location. P0420 is "Catalyst System Efficiency Below Threshold (Bank 1)" and P0430 is the same code for Bank 2. On inline 4-cylinder and inline 6-cylinder engines there is only one bank, so P0430 never appears. On V6, V8, V10, and V12 engines, Bank 1 is the side containing cylinder 1 (often labeled in service manuals and varies by manufacturer), and Bank 2 is the opposite side.
Which side is Bank 2 on a V8 or V6 engine?
Bank 2 is opposite the side containing cylinder 1. On most GM and Ford V engines, Bank 1 is the passenger side and Bank 2 is the driver side. On most Asian (Toyota, Honda, Nissan) and European V engines, Bank 1 is the rear-facing side of a transverse engine, and Bank 2 is the front-facing side. The vehicle service manual confirms the convention for your specific engine.
Can I just replace the rear O2 sensor to fix P0430?
Sometimes. The downstream (rear) O2 sensor on Bank 2 is what generates the P0430 measurement, so a slow or failing sensor can produce a false catalyst code. Replacing it ($100-$300 plus labor) is a reasonable first step if the sensor is original at 100,000+ miles. However, if the catalyst is genuinely degraded, a new sensor will just confirm the problem and the code returns. The diagnostic order: rule out exhaust leaks and upstream misfires, then test the rear O2 sensor response, then suspect the catalyst.
Will P0430 fail emissions inspection?
Yes. Catalyst efficiency codes are emissions-related and any stored DTC triggers automatic failure of OBD-II inspection. Clearing the code immediately before inspection also fails because the catalyst readiness monitor resets to Not Ready. Plan to fix the actual cause (replace catalyst, replace sensor, repair upstream fuel/ignition issue) and complete 100-200 miles of mixed driving before re-testing.
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