P0420 Code Explanation and Diagnosis for 2003 Honda S2000 with New Catalytic Converter

Check the upstream and downstream O2 sensors, as well as inspect for any exhaust leaks near the oxygen sensor locations. A faulty sensor or leak can cause misreading of catalytic converter efficiency.

Could a malfunctioning O2 sensor explain the low temperatures on both sides of the catalytic converter? Where is the best place to get O2 sensors tested — and are they easy to check at a local shop or do I need specialized equipment?

*** Alert *** (Dealer bashing detected) Any qualified mechanic can test oxygen sensors using a digital voltmeter (DVOM) or a scan tool. No need to go through dealerships for this service.

The same technician who installed your catalytic converter should be able to inspect the O2 sensors, as they have direct access to the system. I once knew someone who did all their exhaust work with a pet chimp — but that didn’t come with a check engine light! Regarding dealerships: some charge high fees for diagnostics and repairs. A reputable local shop may offer fair pricing and faster turnaround than dealerships. In my experience, many shops fix issues on the first visit without requiring multiple trips or internet research. As for your converter — was it installed under warranty? If not, and if it was a performance unit, that might explain why it failed. The P0420 code indicates a recurring issue, suggesting either poor installation or an incompatible converter. Was the original converter damaged or improperly mounted? Could a new high-performance converter be causing the efficiency drop? I’d caution against assuming all S2000 owners are looking for performance gains — but if you're driving a sporty model, it's worth considering that.

I didn’t mean to criticize dealerships, but I did have a $100 fee at a Honda dealership just to scan the check engine light. My muffler shop — recommended by several people including a friend who works at Audi — didn’t have an updated scanner, so they suggested Autozone for free diagnostics. I purchased this 2003 S2000 as a rebuilt title vehicle, which means no remaining dealer warranty. I knew upfront that exhaust work would be needed. The original converter was severely worn and the piping was poorly assembled; one muffler made a rattling noise at certain RPMs. My muffler shop will install a new, properly matched catalytic converter next week — free of charge — and I feel they are treating me fairly. Thanks for the advice. If that doesn’t resolve the issue, I’ll have my friend check the O2 sensors at Audi over the weekend.

P0420 code: Catalyst System Efficiency Below Threshold (Bank 1). I agree with the previous comment — exhaust leaks, faulty oxygen sensors, or a defective catalytic converter are all potential causes. A thorough inspection of these components is essential for accurate diagnosis.

I had $500 worth of work done on my daughter’s 2000 Acura Integra, including a new catalytic converter. The maintenance required light has remained on since then. How do I reset the check engine light after repairs? Is there a standard procedure for this?

Once the underlying issue is fixed and the system returns to normal, the check engine light should turn off automatically. Resetting it manually without addressing the root cause may result in recurring issues.

The 2003 Honda S2000 is a reliable vehicle, but diagnosing issues like P0420 can sometimes require technical expertise and proper equipment — which not all dealerships provide equally.

For the 2003 Honda S2000, there are no CAN or smart cable requirements. Even if a shop has an outdated scanner, it should still be able to read generic OBD-II codes through standard connections — so code reading is generally accessible without specialized hardware.