P0420 - Catalyst System Efficiency Below Threshold (Bank 1)
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System
Powertrain (Engine & Gearbox)
Related Fault Codes
Technical Description
The Engine Control Unit (ECU) monitors the catalytic converter's efficiency by comparing the signals from the upstream (pre-cat) and downstream (post-cat) oxygen sensors. A functioning catalyst stores oxygen, resulting in a steady signal from the downstream sensor (typically 0.45V to 0.7V). If the downstream sensor signal mirrors the upstream sensor (oscillating rapidly between 0.1V and 0.9V), it indicates the catalyst has lost its Oxygen Storage Capacity (OSC). When efficiency drops below a calibrated threshold, P0420 is stored.
Possible Causes
Defective or degraded catalytic converter (Bank 1)
Exhaust leak upstream of or near the catalyst
Faulty downstream oxygen sensor (Bank 1 Sensor 2)
Engine misfires or leaking fuel injectors leading to catalyst damage
Symptoms
Recommended Solution
Workflow
Mechanic's Tip
Before replacing expensive components, always double-check wiring and connections. 80% of all sensor faults are actually due to corroded contacts or broken wires.
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