P0420 Fix for Honda Accord & Civic

P0420 (catalyst system efficiency below threshold) is one of the most common check engine codes on the Honda Accord and Civic. It usually comes from an aging catalytic converter that still runs fine but trips the light, or from an aftermarket exhaust or header. Before you spend $600 or more on a new cat, this is the part to try first.

The Fix: 90° O2 Sensor Spacer

The Brockett Built 90° O2 sensor spacer threads into the downstream (rear) O2 bung and moves the sensor out of the direct exhaust stream, so the ECU reads a clean signal and stops throwing the catalyst-efficiency code. CNC-machined for the M18x1.5 thread, the standard Honda O2 size.

• 4-cylinder Accord and Civic are single-bank, so you need one spacer.
• About a 10-minute install. Clear the code and drive a readiness cycle.
• Downstream (rear) sensor only, never the upstream sensor.

One Spacer or Two?

• 4-cyl Accord / Civic (K24, R18, and similar): single bank, one spacer (P0420).
• V6 Accord (J-series): dual bank. If you also have P0430, grab the 2-Pack.

FAQ

Will this hurt my gas mileage?
No. The rear O2 sensor only monitors the cat, it doesn't control fueling, so economy and driveability are unchanged.

Will it clear the code for good?
Yes, if the P0420 is from a lazy or aging cat or an aftermarket exhaust. It won't fix a code caused by a bad O2 sensor or an exhaust leak.

Legal note: designed for modified/off-road use. Check local emissions laws.

Get the 90° O2 Sensor Spacer: $42 →

Machined by Brockett Built in central Illinois. Ships same or next business day. 30-day fitment guarantee.