Answer
Sep 04, 2025 - 09:09 PM
P0420 means “Catalyst System Efficiency Below Threshold (Bank 1)” — in plain terms, your car thinks the catalytic converter isn’t doing its job properly. Here’s how to go about it:
First, some quick checks:
- Make sure it’s not a false alarm — clear the code and see if it comes back. Sometimes it trips from a temporary glitch.
- Check for exhaust leaks — especially before the catalytic converter. Even a small leak can mess with sensor readings.
Most common causes:
- Worn-out catalytic converter – it’s doing less filtering, so the downstream O2 sensor picks it up.
- Lazy downstream O2 sensor – could be sending bad signals even if the cat’s still good.
- Bad upstream O2 sensor – messing up the fuel mixture and making the cat work too hard.
- Rich or lean running engine – misfires, vacuum leaks, or fuel injector problems can damage the cat over time.
What you can do:
- Start by inspecting both O2 sensors — upstream and downstream. If they’re original and have high mileage, it’s worth replacing the downstream one first.
- If sensors check out but the code keeps coming back, the catalytic converter itself might be on its way out.
- Also check for any other codes (like misfire or fuel trim issues), because P0420 can be a side effect of those.
Of course, the service manual provides a more detailed procedure for fixing this code. Given the limited information, we're unable to offer much accurate troubleshooting advice.
If you could provide us with more details about your 2018 Sportage (e.g., engine, generation, market, etc.), we'll gladly help point you in the right direction.
