Summary
P0420 indicates the catalytic converter on Bank 1 is not reducing emissions efficiently enough. The most common cause is a failing catalytic converter (45% of cases), followed by a faulty downstream O2 sensor (25%) and exhaust leaks (15%). Before replacing the catalytic converter ($800–$2,500), check both O2 sensors and inspect for exhaust leaks — the fix is often a $25–$120 sensor swap.
Severity: Moderate — not immediately dangerous
Safe to drive: Yes, with caution — emissions will increase and fuel economy may drop
Repair cost: $25–$2,500 depending on cause
DIY difficulty: Moderate to Advanced
What does P0420 mean?
Your vehicle's powertrain control module (PCM) monitors catalytic converter efficiency by comparing the upstream (pre-cat) and downstream (post-cat) oxygen sensors on Bank 1. A healthy catalytic converter creates a clear difference between these two readings — the upstream sensor fluctuates rapidly while the downstream stays relatively steady.
When the PCM sees the downstream O2 sensor start mimicking the upstream sensor's pattern, it means the catalytic converter isn't doing its job. The converter's internal catalyst material has degraded to the point where exhaust gases pass through without adequate chemical conversion. The PCM sets P0420 after running this comparison over multiple drive cycles.
Bank 1 is the side of the engine containing cylinder 1. On inline engines, there's only one bank. On V6 and V8 engines, Bank 1 is typically the side closer to the front of the engine (check your service manual — this varies by manufacturer).