Code Definition

P0420 — Catalyst System Efficiency Below Threshold (Bank 1)

System: Powertrain  |  Type: Generic (OBD-II Standard)  |  Make: Generic

About P0420

P0420 is a generic (obd-ii standard) powertrain diagnostic trouble code. It belongs to the P-prefix family of codes, which covers engine, transmission, emissions, or fuel-system faults.

When P0420 is stored, the module has detected a signal or supply voltage lower than the expected operating range; and a bank-specific fault, meaning it applies only to one cylinder bank of a multi-bank engine.

Scanning & clearing: A generic OBD-II scan tool will usually read and clear powertrain (P) codes. For live data, freeze-frame capture, and bi-directional tests you may need a more capable scan tool that supports the vehicle's ECM protocol.

Because P0420 is a generic OBD-II code, its base definition is shared across manufacturers. The specific diagnostic steps, reference values, and repair procedures can vary by vehicle, and the TaT Diagnostic Assistant includes verified procedures submitted by working technicians.

Code Information

The powertrain control module (PCM) sets P0420 when it detects that the catalytic converter on Bank 1 is not operating at the expected efficiency level. This is determined by comparing the signals from the upstream (pre-catalyst) and downstream (post-catalyst) oxygen sensors. If the post-catalyst sensor signal closely resembles the pre-catalyst sensor, it indicates that the converter is not effectively storing and converting exhaust gases, triggering the fault.

Quick Facts