So your check engine light is illuminated, and your code reader is spitting this out at ya:
DTC P0420 Catalyst System Efficiency Below Threshold (Bank1)
What, exactly, does this code mean? Is your catalytic converter at the end of it’s service life? The short answer here is Yes. Your converter is likely to be at fault for this code. Your vehicles On Board Diagnostics (OBDII) system monitors the upstream and downstream Oxygen Sensor readings and compares the switching of a rich-lean condition between the two.
The “upstream” O2 sensor, located in the exhaust stream before the converter, monitors the amount of oxygen in the exhaust gas as it leaves the engine while the “downstream” O2 sensor, usually located behind the cat or even in it, monitors the amount of oxygen in the exhaust gas after it passes through the converter.
When you first start your vehicle and the engine is cold, the upstream O2 sensor as well as the downstream O2 sensor will likely have about the same readings, as the gasses are not yet hot enough. Typically, the upstream O2 sensor changes from rich to lean very often as the engine computer adjusts the air/fuel mixture. However, when your vehicle reaches operating temperature, the exhaust gasses passing through the converter are hot enough to begin reacting with the metals inside of the converter ( platinum, rhodium and/or palladium – in some newer converters, GOLD! – Gold could actually increase oxidation, reducing pollutants even more!) and the downstream O2 sensor stays more so on the lean side.
Of course, that’s during normal operation. When the vehicle is at operating temperature and the downstream O2 sensor’s switching activity doesn’t tone down as per usual, that’s essentially telling the OBDII system that there is not enough oxygen coming out of the “cat”, so the converter isn’t properly oxidizing the hydrocarbons passing through it, the system sets a P0420 code and commands your malfunction indicator lamp on.
From here, you have few options if you want your Check engine light to go out – Replacement of your catalytic converter is looking more like the solution. In order to properly diagnose this issue, it is recommended that you bring your vehicle into a shop and mention to them your P0420 trouble code, allow that shop to do some trouble shooting and determine the cause and the fix.
However, for the DIY crowd, there are a couple of things you can check for before determining which action to take. Check for failure of the heated oxygen sensor(s) as well as an exhaust leak. If a faulty O2 Sensor is found, replace it. If you find an exhaust leak, repair it. Another possible cause of this code is a faulty PCM (Powertrain control Module). It can be difficult, if not complex, to test for these issues and a repair facility is recommended if you do not have the proper tools/instruments at hand.