A P0420 code on your 2016 Honda HR-V indicates the catalytic converter isn't converting exhaust pollutants as efficiently as required. This emissions-related code must be addressed for emissions testing compliance and to ensure your vehicle meets environmental standards.
Understanding P0420
The PCM monitors catalytic converter efficiency by comparing oxygen sensor readings before and after the converter. The upstream (pre-cat) sensor should show rapid fluctuation as the engine cycles between rich and lean. The downstream (post-cat) sensor should show relatively steady readings if the converter is processing exhaust properly. When downstream sensor activity mirrors upstream activity, the converter isn't working and P0420 sets.
Why HR-V Converters Fail
The compact 1.8-liter engine in the HR-V produces less exhaust heat than larger engines, which can make catalyst warm-up slower. Short trips that don't allow the converter to reach optimal operating temperature (around 800°F) reduce efficiency. Oil consumption, coolant leaks, or running rich poison the catalyst material. Physical damage from road debris or internal breakdown from age can also cause failure.
Diagnosis Before Replacement
Don't replace the converter without confirming it's actually failed. Check for exhaust leaks between the engine and downstream sensor that could affect readings. Test both oxygen sensors for proper response—a lazy or failed downstream sensor can falsely trigger P0420. Monitor fuel trim data; if the engine runs rich or lean, address that first as it may be the root cause. Check for misfire codes that would indicate unburned fuel damaging the converter.
Oxygen Sensor Testing
Using live data, observe the upstream sensor during steady idle—it should switch between approximately 0.1V and 0.9V several times per second. The downstream sensor should hover relatively steady, usually between 0.5V and 0.8V. If the downstream sensor mimics the upstream sensor's switching pattern, the converter is failing. If sensor response seems abnormal (stuck, slow, or erratic), the sensor may be at fault rather than the converter.
Repair Options
Oxygen sensor replacement is relatively inexpensive at $150-$300 per sensor and should be ruled out first. Catalytic converter replacement for the HR-V costs $500-$1,000 for a quality aftermarket unit or $800-$1,500 for OEM. California and some other states require CARB-certified converters that cost more but ensure emissions compliance. After any repair, the code must be cleared and the catalyst monitor must complete successfully before emissions testing.