Chrysler 300C P0430 (2005–2010): Catalyst 2/1 Efficiency Below Threshold — Diagnostic Guide

Chrysler-specific code — factory diagnostic data

P0430 means the PCM’s State of Change (SOC) catalyst monitor has determined that the Bank 2 catalytic converter is no longer storing enough oxygen to keep exhaust-gas breakthrough below the emissions-certification threshold. The monitor works by comparing the upstream and downstream O2 sensor signals: a healthy catalyst holds oxygen long enough that the downstream sensor reads a steady, near-stoichiometric value; a worn catalyst lets oxygen pass through, causing the downstream sensor to mirror the upstream’s fast lean/rich switching. The higher that downstream SOC index climbs, the more exhaust gas is escaping the cat untreated. P0430 is a two-trip fault — the PCM requires two consecutive drive cycles with the fault present before illuminating the MIL, and three good drive cycles to turn it off.

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P0430 Chrysler 300C Quick Answer

Don’t replace the catalytic converter first. The factory procedure puts the catalyst as the last suspect, after ruling out exhaust leaks between the engine and the O2 sensor, engine mechanical issues that could be burning oil or coolant into the cat, and an aging front (upstream) O2 sensor — especially if the rear O2 sensor was recently replaced without also replacing the front. That mismatch is one of the explicit notes in Chrysler’s own diagnostic path. Also check for concurrent O2 sensor DTCs and for contaminated fuel, silicone, oil or coolant before ordering a cat.

Last updated: April 19, 2026

The Diagnostic Procedure

Tools: scan tool with live data and freeze-frame, visual inspection mirror/light, smoke machine (Miller 8404A Evaporative Emissions Leak Detector or equivalent for exhaust leak testing). Warm the engine to full operating temperature before any live-data work.

1. Confirm the DTC is active. Ignition ON, engine not running. Read DTCs with a scan tool.
   → P0430 active: proceed to step 2.
   → P0430 pending or history only: intermittent. Follow the intermittent-condition procedure. After any repair, perform the Powertrain Verification Test.
2. Visually inspect the Bank 2 catalytic converter. Ensure top AND bottom are thoroughly inspected. Look for dents or holes, severe discoloration caused by overheating, internal breakage (rattle when tapped), or leaks at the pipe junctions.
   → Physical damage found: replace the cat AND repair the condition that caused the failure — don’t replace the cat without fixing the root cause or the new cat will fail too.
   → Cat looks OK: proceed to step 3.
3. Exhaust leak inspection. Start the engine. Inspect the exhaust for leaks between the engine and the O2 sensor. If a leak is heard but can’t be located, use the Miller 8404A EELD (or equivalent smoke tester) on the exhaust: connect the smoke supply to the tail pipe, apply smoke, follow the exhaust path with a white light looking for exiting smoke. For concealed leaks, use the UV black light and yellow goggles to look for residual dye left by the smoke (bright green or bright yellow under UV). Be sure to check the exhaust manifold-to-cylinder-head connection.
   → Leak found: repair or replace the leaking parts.
   → No leak: proceed to step 4.
4. Engine mechanical condition check. Check the exhaust for excessive smoke caused by an internal engine problem (oil burning, coolant ingestion, severe rich-running). The factory procedure calls out engine mechanical condition as a cause that must be ruled out before condemning the catalyst.
   → Mechanical issue present: repair before replacing the cat. A contaminated catalyst will not recover after the root cause is fixed.
   → No mechanical issues: proceed to step 5.
5. Review repair history for O2 sensor replacement. Ask the owner or check records: has the rear (Bank 2 Sensor 2) O2 sensor been replaced without also replacing the front (Bank 2 Sensor 1) upstream sensor?
   → Yes — only rear was replaced: the factory note flags this as a likely cause. Replace the Bank 2 Sensor 1 (upstream) O2 sensor.
   → No: proceed to step 6.
6. Replace the Bank 2 catalytic converter. With exhaust, engine mechanical condition, and O2 sensor pairing ruled out, the catalyst itself is the remaining cause. After replacement, perform the Powertrain Verification Test to confirm the monitor passes.

Professional tip. On the 5.7L HEMI with MDS (Multi-Displacement System), the Bank 2 catalyst sees variable thermal load as half the cylinders drop out during light-load cruising — this is normal and doesn’t damage the catalyst. If any MDS-related DTCs are stored alongside P0430, diagnose those first; asymmetric MDS operation can accelerate catalyst aging on the affected bank.

Common Causes on LX Platform Vehicles

The factory procedure lists four possible causes, ranked here roughly by how often each is confirmed in real-world diagnosis, and mapped to the step that isolates each:

• Aging O2 sensor — isolated by step 5. The “rear replaced without front” mismatch is called out as an explicit factory note; it accounts for a significant portion of P0430 cases where the catalyst itself is healthy.
• Exhaust leak upstream of the Bank 2 sensors — isolated by step 3. Unmetered air entering the exhaust upstream of the sensors makes the downstream sensor read lean and fools the catalyst monitor into flagging a healthy cat as failed. Check the manifold-to-head connection carefully.
• Engine mechanical condition — isolated by step 4. Oil burning, coolant ingestion, or severe rich-running all push contaminants or excessive heat into the catalyst. The factory procedure explicitly warns that repairing the catalyst without fixing the mechanical cause will destroy the replacement too.
• Contamination (from step 2 visual / step 4 root cause) — contaminated fuel, unapproved silicone, oil, and coolant are all called out in the factory notes as items that damage both the O2 sensors and the catalyst.
• Genuine catalytic converter failure — isolated by step 6, after all upstream causes are ruled out. This is the repair the factory treats as the last option.
• Concurrent misfire damaging the catalyst — not ranked in the factory causes list but diagnostic common sense: unburned fuel from a running misfire is a primary catalyst killer. If any misfire DTC is stored or was recently cleared, diagnose that first.
• Concurrent rich-running fault — again, not in the factory’s four-item list but diagnostically relevant: prolonged rich operation thermally destroys the catalyst. Diagnose any stored fuel-system DTCs before considering the catalyst itself.

Severity & Driving

P0430 by itself is not a safety emergency. The engine runs normally, fuel economy is roughly unchanged, and there is no immediate driveability impact. The practical consequences are emissions-test failure (the car will not pass state inspection in most US states with P0430 stored) and slow-burn long-term damage: a degraded catalyst runs hotter than normal, which can eventually damage the downstream exhaust and the downstream O2 sensor. Driving to a workshop is fine, but don’t defer past the next registration or inspection date. If the MIL is flashing (not just solid), that indicates active catalyst-damaging misfire rather than a cat efficiency code — stop driving and diagnose the misfire first.