WOMS
P code emissions Medium severity

P0420 Catalyst System Efficiency Below Threshold (Bank 1)

The downstream oxygen sensor on Bank 1 is reporting oxygen levels that indicate the catalytic converter is no longer effectively reducing emissions.

Definition

P0420 is set when the PCM compares the upstream and downstream O2 sensor signals on Bank 1 and determines the catalyst is not storing oxygen efficiently. In a healthy system the downstream O2 sensor reads a much flatter signal than the upstream sensor. As the catalyst ages or is contaminated, the downstream sensor begins to track the upstream sensor, which the PCM interprets as a loss of conversion efficiency.

A P0420 will trigger the MIL (Check Engine Light). It is an emissions-related code that will cause a vehicle to fail an OBD2 emissions inspection.

Common causes

  • Failed or degraded catalytic converter (Bank 1)
  • Failed downstream (post-cat) oxygen sensor
  • Exhaust leak upstream of the post-cat O2 sensor
  • Engine running rich or lean (long-term fuel trim out of spec)
  • Coolant or oil contamination of the catalyst
  • Aftermarket high-flow cat or test pipe
  • Damaged or burned-out catalyst substrate from misfires

Common misdiagnoses

  • Replacing the catalytic converter without verifying the root cause (a failing O2 sensor or rich/lean condition will quickly kill the new cat).
  • Replacing both O2 sensors when only one is at fault.
  • Ignoring exhaust leaks at the manifold or pre-cat flange.

Troubleshooting steps

  1. 1. Read all stored codes

    Check for misfire (P0300-P03xx), fuel trim (P0171/P0172/P0174/P0175), or O2 sensor codes that should be resolved first.

  2. 2. Inspect for exhaust leaks

    Visually and audibly check the exhaust manifold, gaskets, and pre-cat flange. A leak upstream of the post-cat O2 sensor lets fresh air skew the reading.

  3. 3. Compare O2 sensor waveforms

    With a scan tool, view Bank 1 Sensor 1 and Sensor 2 live data. Sensor 1 should swing 0.1V-0.9V; Sensor 2 should hold a steadier voltage around 0.6V-0.8V. If Sensor 2 mirrors Sensor 1, the catalyst is degraded or Sensor 2 is faulty.

  4. 4. Check long-term fuel trim

    LTFT should be within ±10%. Persistent rich/lean trims must be resolved before condemning the cat.

  5. 5. Test downstream O2 sensor

    If Sensor 2 voltage is stuck or swinging widely, swap with Bank 2 sensor (on V-engines) and see if the code follows.

  6. 6. Replace catalyst as last step

    After exhaust, fuel trim, and O2 sensors are verified, replace the Bank 1 catalyst with an OE-equivalent unit.

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