The OBD-II trouble code P0420 means that the catalytic converter on Bank 1 is not working as efficiently as it should be. Bank 1 refers to the side of the engine with cylinder 1.
🔧 P0420 – Catalyst System Efficiency Below Threshold (Bank 1)
💡 What It Means:
This code indicates that the catalytic converter on Bank 1 is not working as efficiently as it should be. Bank 1 refers to the side of the engine with cylinder 1.
The engine control module (ECM) uses data from the upstream (pre-cat) and downstream (post-cat) oxygen sensors to monitor how well the catalytic converter is cleaning exhaust gases. If the post-cat sensor shows similar readings to the pre-cat sensor, it means the cat isn’t doing its job well.
🛠️ Common Causes:
| Cause | Description |
|---|---|
| 🔧 Bad catalytic converter | Most common cause – worn out or damaged cat can’t clean emissions. |
| 🔌 Faulty O2 sensor (usually downstream) | Sends incorrect data to ECM, triggering false code. |
| 🔥 Engine misfire or rich/lean condition | Unburned fuel or air/fuel imbalance can damage cat. |
| 💨 Exhaust leaks | Leak before O2 sensor can throw off sensor readings. |
| 💻 ECM software issue | Rare, but possible if all hardware checks out. |
🔍 Symptoms:
Check Engine Light (CEL) on
Often no noticeable drivability issues
Failed emissions test
Sometimes decreased fuel efficiency
Also read : Fixed : Marvel Rivals Error Code 211
🧪 How to Diagnose:
Scan live data from O2 sensors:
The upstream O2 sensor (pre-cat) should fluctuate.
The downstream O2 sensor (post-cat) should show a stable voltage.
If both fluctuate the same → cat likely bad.
Check for exhaust leaks around the manifold, flex pipe, and cat.
Inspect and test O2 sensors, especially the downstream sensor.
Check fuel trims and misfire data (codes like P0300–P0306) that may point to rich/lean conditions.
✅ Fixes:
| Fix | Cost (approx.) |
|---|---|
| Replace catalytic converter | $400–$2,000+ (OEM cats are expensive) |
| Replace downstream O2 sensor | $50–$150 |
| Fix exhaust leak | $50–$300 |
| Repair engine misfire or fuel issue | Varies widely depending on root cause |
| Reprogram ECM (rare) | $100–$300 |
⚠️ Don’t clear the code and ignore it. If the cat is failing, it could lead to higher emissions or eventual performance issues.
🧪 Temporary Workaround:
Some people use O2 sensor spacers or simulators to bypass the code, but this is not recommended for emissions-legal vehicles and can result in fines or failed inspections.
Be the first to comment