Miata Exhaust CEL P0420
This is a common issue we see these days with OBDII cars and aftermarket exhaust systems. Some systems are very finicky about properly sensing the airflow going through the exhaust system, that if they sense too much air at the downstream oxygen sensor, it will trigger a check engine light. This can be caused with a larger diameter exhaust, high flow catalytic converter, or a catalytic converter delete exhaust.
This miata came in due to this issue after the owner installed an aftermarket exhaust. The dealer told him the stock exhaust needed to go back on or else there was no other fix. But we disagree 😉
The goal is to trick the downstream O2 sensor into seeing a reduced amount of air flow coming past it. We use a simple plug that restricts the direct flow on the sensor.
Modified. The tip of the o2 sensor will have enough area around it to read a reduced flow.
And installed inline with the rear o2. No more CEL.