P0420

Catalyst System Efficiency Below Threshold (Bank 1)

Symptoms

• •Check engine light
• •Possible mild power or economy change
• •May fail an emissions test

Likely causes

• •Aging catalytic converter
• •Faulty downstream oxygen sensor
• •Exhaust leak
• •Engine running rich or lean

Can I drive?

Plan a workshop visit soon; not usually an emergency.

Estimated repair cost (Malaysia)

RM300–RM1,200 (approximate)

Diagnostic guide (for mechanics)

Work through these in order — cheapest and safest checks first, before replacing expensive parts.

1. 1. Scan for other codes first

   Clear unrelated faults and confirm there are no misfire, fuel-trim, or O2-sensor codes. Those must be fixed before judging the catalytic converter.

2. 2. Check for exhaust leaks

   Inspect the exhaust between the engine and the rear oxygen sensor. A leak upsets the downstream reading and mimics a failed catalyst.

   Test: Visual and audible inspection at idle; soapy-water on joints.

   Expected: No leaks before the downstream (rear) O2 sensor.

3. 3. Compare upstream vs downstream O2 sensors

   On a warm engine at steady cruise, read both bank-1 oxygen sensors on live data.

   Test: Live data: upstream (sensor 1) vs downstream (sensor 2) voltage.

   Expected: Upstream swings 0.1–0.9 V; a healthy catalyst keeps downstream nearly flat near 0.6–0.7 V.

4. 4. Only then consider the catalytic converter

   If fueling, leaks, and sensors are all good and the downstream sensor mirrors the upstream, the catalyst is likely degraded.

Tools: OBD-II scan tool · Vehicle on a steady test drive

When to escalate: Confirm catalyst failure before replacement — a converter is far costlier than a sensor or gasket. Send for specialist confirmation if readings are borderline.

This is general information, not a diagnosis. AutoMalaya OBD helps explain your car’s actual scan data; a qualified mechanic may still need to inspect the vehicle.

Related fault codes

See sample report