P0420 – Catalyst System Efficiency Below Threshold (Bank 1) – (Dodge / Chrysler)

Randy Worner

By Randy Worner (Contact Me)
Last Updated 2020-12-14
ASE/ACA World Class Technician

CodeFault LocationProbable Cause
P0420 Catalyst System Efficiency Below Threshold (Bank 1)
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Catalytic converter, wiring, HO2S 2

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What Does Code P0420 Mean?

This generic powertrain/engine diagnostic trouble code typically applies to most manufacturers since 1996. 

This code is focused on the voltage being returned by the Oxygen Sensors (O2S), both Pre-catalytic Converter (upstream) and Post- Catalytic Converter (downstream) to the engine controller (ECM/PCM). The PCM monitors the voltage and the sensor’s ability to send both a low voltage (typically below 2.5V) and a high voltage (above 2.5V). When the engine is started cold, both O2 sensors should produce the same voltage. Once the engine warms up and more importantly, the catalytic converter, the upstream O2 sensor should switch above and below approximately 2.5 volts. The downstream O2 sensor voltage should begin to become more stable, less fluctuating. This stable downstream O2 sensor voltage is due to the catalytic converter slowly releasing oxygen into the exhaust stream as it is being utilized. The downstream O2 sensor voltage should not switch as fast. It will look as if it is stuck around one set voltage. If the voltage on the downstream O2 sensor mirrors the upstream O2 sensor, or the voltage varies too much, a fault code is stored. This may be a mechanical issue (failed catalytic converter) or an electrical fault (sensor/signal issue). Always remember, check the engine for any type of misfire, as this will lead to premature catalytic converter failure.

Where is the P0420 sensor located?

On most Dodge/Chrysler OHC engines the Oxygen Sensor is screwed into the front exhaust manifold. On OHV or Cam-in-Block engines, the Oxygen Sensor is screwed into the exhaust manifold that attaches to the head that has the number cylinder 1 in it. The downstream O2 sensor will be in the exhaust system after the catalytic converter. 

Typically, the causes for this code to set are:

Most common:

  • Failed Catalytic Converter
  • Oxygen sensor – Downstream O2 sensor signal circuit voltage shorted to power or ground, failed O2 sensor

Contributing Factors

  • Fuel system components – including injectors and fuel quality
  • Worn ignition system components, to include the coil, plug wires, spark plugs, etc.
  • (Misfires allows the unburned fuel and air to escape to the exhaust system. Misfires cause O2 voltage to stay low due to high oxygen content coming out of the cylinder.)

Last possibility 

  • Failed PCM – very rarely

What are the common causes of code P0420?

Specific to Chrysler products, these are the more common issues with the Catalyst System Efficiency Below Threshold (Bank 1) fault code:

  • Dodge Avengers w/2.4L – Failed catalytic converters and oxygen sensors are the most common issue related to P0420.
  • Dodge Avengers w/3.6L – Failed catalytic converters and oxygen sensors are the most common issue related to P0420, with a cracked exhaust manifold next.
  • Chrysler 300, Dodge Charger/Challenger w/2.7L/3.6L/5.7L – Failed catalytic converters and downstream oxygen sensors are the most common issue related to P0420, with upstream O2 sensors a close second on the 2.7L engines.
  • Chrysler Minivans w/3.6L – Failed catalytic converters and oxygen sensors are the most common issue related to P0420, with a cracked exhaust manifold next.
  • Dodge Durango/Dodge Journey/Jeep Wrangler and Grand Cherokee w/3.6L – Failed catalytic converters and oxygen sensors are the most common issue related to P0420, with a cracked exhaust manifold next.
  • Dodge Dart w/2.0L – Failed catalytic converters and oxygen sensors are the most common issue related to P0420.
  • Jeep Compass/Patriot w/2.0L or 2.4L – Failed catalytic converters and oxygen sensors are the most common issue related to P0420.

In all cases, high quality OEM approved catalytic converters are the best choice. Low cost aftermarket alternatives may need to be replaced before the end of their warranty.