My '06 H3 was giving me an P0420 so I replaced the manifold/converter. After I replaced it, I'm still getting the same code. My downstream O2 sensor is almost mimicing my upstream sensor instead of being a steady reading. Is it possible the new manifold has a bad converter on it (it a cheap ebay manifold) or could it be my downstream O2 sensor that's bad?

 

the new converter might not have enough catalist material or is a straight through header.

 

Check for any leak(s) upstream from the downstream 02 sensor. I assume you cleared the code after the manifold swap. Those will linger if you did not.

Next, 02 Sensors are pretty cheap, quick and easy to replace. Might try that and see what it gets you.

 

HUMMER DTC P0420

P0420 code indicates the "Catalyst System Efficiency Below Threshold (Bank 1),
Common Causes

• Catalytic Converter: Damaged or worn-out catalytic converter.
• Oxygen Sensors: Faulty rear (downstream) oxygen sensor.
• Exhaust Leaks: Leaks in the exhaust system near the sensor.
• Engine Issues: Misfires, fuel injector issues, or engine running too rich/lean.