The P420 Obdii code indicates “Catalyst System Efficiency Below Threshold (Bank 1).” This means your vehicle’s computer believes the catalytic converter isn’t working correctly. Since the converter lacks internal sensors, the computer relies on oxygen (O2) sensors located before and after the catalytic converter to assess its performance. A common cause of the P420 code is a faulty downstream O2 sensor (Bank 1 sensor). These sensors can provide inaccurate readings, triggering the code. Replacing an O2 sensor is typically less expensive than replacing a catalytic converter, costing between $80 and $125 plus labor.
A malfunctioning internal coolant temperature sensor can also trigger a P420 code. If the sensor incorrectly reports a higher coolant temperature, the engine’s cooling fans will operate more frequently, keeping the engine cooler than optimal. A cooler engine produces less efficient emissions, hindering the catalytic converter’s ability to reach its required operating temperature and potentially triggering the P420 code. Replacing this sensor is also relatively inexpensive. Dealerships can test these sensors with a voltmeter to diagnose the issue accurately.
Finally, original catalytic converters (and pre-cats) often have an 8-year/80,000-mile warranty under federal law. If your vehicle falls within this timeframe and mileage, the repair might be covered under warranty. However, O2 sensors typically have a shorter warranty period of 4 years/50,000 miles.
