P0420 and P0430 Codes Persisting on 2009 Chevrolet Silverado 5.3L After Cat Replacement
I'm completely stuck trying to resolve persistent P0420 and P0430 diagnostic codes on my 2009 Chevrolet Silverado 5.3L. Both catalytic converters were replaced at a muffler shop, and they performed an O2 sensor smoke test with no leaks detected. No other error codes are present now, though I previously had an EVAP code that cleared itself. The P0420 and P0430 codes reappear consistently after driving approximately 185 miles, suggesting a recurring drive cycle issue. I've checked fuel trims—both short-term and long-term—are well within normal ranges (below 6 on both banks), and the fuel rail pressure is stable at 42–44 psi. The engine is stock with no modifications, rebuilt at 187,000 miles using aftermarket O2 sensors (Bosch). The muffler shop installed aftermarket catalytic converters due to concerns about temperature variation between front and rear of the exhaust system. I'm wondering if there could be an underlying issue such as a vacuum leak or engine operating condition that affects catalytic converter performance. Any insights into how to properly diagnose these codes—especially in relation to oxygen sensor readings, fuel trim behavior, or exhaust leaks? Thanks for any help.
To assess catalytic converter effectiveness, compare pre- and post-cat O2 sensor voltages. Engine stockness matters—ensure the engine reaches proper operating temperature without aftermarket modifications or non-OEM sensors, as these can skew readings. If original converters failed, were there any known causes such as fluid loss, performance mods, or unresolved issues? A vacuum leak could also trigger P0420/P0430 codes.