MG ZS (Gen2) P0420 Catalyst System Efficiency Below Threshold

The P0420 Catalyst System Efficiency Below Threshold fault indicates that the engine control module (ECM) has determined the catalytic converter is no longer reducing exhaust emissions efficiently on Bank 1. The ECM detects this condition by comparing the switching patterns of the upstream and downstream oxygen sensors during closed-loop operation. If both sensors display similar readings, the catalytic converter is considered to have insufficient oxygen storage capacity and catalyst efficiency. This fault applies to the MG ZS Gen2 (ZS11 platform), introduced from 2024 onwards.

Symptoms

  • Check Engine Light illuminated
  • Reduced fuel economy
  • Increased exhaust emissions
  • Failed emissions inspection
  • Reduced engine performance (in some cases)
  • Sulphur or rotten egg smell from the exhaust
  • Poor throttle response
  • Additional oxygen sensor or fuel system fault codes may be present

Possible Causes

  • Deteriorated or damaged catalytic converter
  • Faulty downstream oxygen (O2) sensor
  • Faulty upstream oxygen (O2) sensor
  • Exhaust leak before or near the catalytic converter
  • Engine running rich or lean for an extended period
  • Engine misfires damaging the catalytic converter
  • Faulty fuel injector causing excessive fuel delivery
  • Oil or coolant contamination of the catalyst
  • Damaged oxygen sensor wiring or connectors
  • Engine control module (ECM) fault (rare)

Diagnosis

Diagnosis begins by confirming the stored fault code and reviewing freeze-frame data using a compatible diagnostic scan tool. Live data from the upstream and downstream oxygen sensors should be analysed to evaluate catalytic converter efficiency, while the exhaust system should be inspected for leaks or damage. The engine should also be checked for underlying faults such as misfires, incorrect fuel trim, injector problems, or oxygen sensor failures that may have contributed to catalyst deterioration. Manufacturer diagnostic procedures may include catalyst efficiency testing, exhaust backpressure measurements, and verification of oxygen sensor performance.

For vehicles operating in New Zealand, accurately identifying the root cause is essential because replacing the catalytic converter without correcting the underlying engine fault often results in repeat catalyst failure.

Accurately diagnosing this fault often requires manufacturer testing procedures, wiring diagrams, connector pinouts, oxygen sensor specifications, exhaust system testing procedures, catalyst efficiency data, and system calibration information. Access to the factory service manual significantly reduces diagnostic time and helps prevent unnecessary catalytic converter replacement.

Applicable Years

2024, 2025, 2026, onwards

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