P0430

What the code means

P0430 sets when the PCM determines that the Bank 2 catalytic converter no longer stores oxygen efficiently enough. The related code P0420 covers Bank 1; P0430 covers Bank 2.

The code doesn't tell you why efficiency dropped, which is why it's often misdiagnosed: shops replace oxygen sensors, then the converters, and the code comes back. Most converters fail because something else killed them first. If you don't find the root cause, the replacement will fail too.

Before you blame the converter, check fuel control (fuel trims), misfire data (current and stored), exhaust leaks, oxygen sensor behavior, and contamination risks (oil, coolant).

Symptoms

• The PCM flags that the converter isn't storing oxygen efficiently — that's P0430 on Bank 2.
• The downstream oxygen sensor starts mirroring the upstream sensor instead of holding a relatively steady voltage in a narrow range — a red flag for converter efficiency.
• Codes or data for fuel trims, misfires, coolant temperature, or oxygen sensors that can skew the picture.
• Misfires, including intermittent ones, on the same bank: raw fuel gets dumped into the exhaust and overheats the converter substrate, destroying its oxygen-storage capacity.
• A small exhaust leak upstream of the downstream oxygen sensor: extra oxygen fools the PCM, and the code sets even with a healthy converter.
• Oil or coolant contamination: oil ash and antifreeze residue coat the substrate and cut efficiency. Causes include overfilling the oil, oil consumption, a faulty PCV system, and internal coolant leaks.

Diagnosis and repair

Step 1. Pull the code and check for related faults

Read the code, confirm which bank is setting the fault, and check related codes: fuel trims, misfires, coolant temperature, oxygen sensors.

Step 2. Check the ECM calibration

Make sure the ECM has the latest calibration. Software updates and TSBs matter: revised logic changes how aggressively the PCM runs the catalyst efficiency monitors.

Step 3. Check fuel control

Combined short-term and long-term trims on the affected bank should be within about ±10%. With a lean or rich mixture, the converter can't store oxygen correctly.

Step 4. Graph the oxygen sensors

The upstream sensor should switch quickly; the downstream sensor on a healthy converter stays relatively steady. The downstream sensor mirroring the upstream signal doesn't prove the converter is bad on its own — you still have to prove the cause.

Step 5. Check misfires

Check current and stored misfire data. If you find misfires on the same bank as P0430, stop: replacing the converter without fixing the misfires guarantees the same fault comes back.

Step 6. Inspect the exhaust system

Physically inspect the exhaust for leaks upstream of the downstream oxygen sensor — one of the simplest checks and one of the most often skipped.

Step 7. Check the oil, PCV, and cooling system

Check oil level and condition, and the PCV system: overfilling the oil, oil consumption, and a faulty PCV poison the converter. Pressure-test the cooling system: internal coolant leaks kill converters and trigger P0420/P0430.

Step 8. Replace the converter

If contamination is present and hasn't been fixed, a new converter won't last. Getting the right converter type matters when you replace it.