P0420 — Catalyst System Efficiency Below Threshold (Bank 1)
P0420 means your catalytic converter isn't cleaning exhaust gases as effectively as it should. It is usually safe to drive short-term, but it will fail an emissions test.
The vehicle will usually drive normally, but an underlying engine fault will destroy the converter if it is left unrepaired. This code will cause an emissions test failure.
What P0420 means
The engine control module compares readings from the oxygen sensors before and after the catalytic converter. On a healthy converter the downstream signal is slow and steady while the upstream one oscillates rapidly. When the downstream signal begins to mirror the upstream one, the module concludes the converter is no longer storing and releasing oxygen, and sets P0420 after the monitor has failed on two consecutive drive cycles.
| System | Emission control |
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Symptoms you may notice
What causes P0420
Catalytic converter has lost efficiency
The ceramic substrate inside the converter is contaminated, melted, or broken down with age, so it no longer stores and releases oxygen. This is the most common cause on vehicles past roughly 150,000 km.
Downstream oxygen sensor is slow to respond
An ageing rear oxygen sensor reports a signal that resembles converter failure even when the converter is healthy. Replacing the converter without testing the sensor is the single most expensive misdiagnosis on this code.
Common misdiagnosis
Compare upstream and downstream sensor traces on a live-data graph before condemning the converter.
Exhaust leak ahead of the rear sensor
A leaking gasket or cracked pipe upstream of the downstream sensor lets ambient air into the exhaust stream, which skews the reading toward lean and mimics a failed converter.
Untreated misfire or rich running condition
Raw fuel reaching the converter overheats it and destroys the substrate. A misfire or fuel-trim fault that was ignored is often the reason a converter failed in the first place.
How to diagnose it
Read every stored code before touching anything
Scan for all stored and pending codes. Misfire, fuel-trim, and oxygen-sensor codes stored alongside P0420 point at the real fault and must be repaired first.
Graph both oxygen sensors at steady cruise
Bring the engine to full operating temperature and watch both sensor voltages live at a steady 2500 rpm. The upstream signal should swing rapidly between roughly 0.1 V and 0.9 V; the downstream signal should stay nearly flat around 0.6 V.
Inspect the exhaust for leaks ahead of the rear sensor
With the engine cold and safely supported, inspect the manifold, flex pipe, and every joint before the downstream sensor for soot trails or cracks.
Safety
Never work under a vehicle supported only by a jack, and let the exhaust cool completely before touching it.
Confirm with a temperature differential test
With the engine warm, measure the pipe temperature at the converter inlet and outlet with an infrared thermometer. A working converter runs hotter at the outlet because the reaction inside it generates heat.
Vehicle-specific guides
Toyota
Related codes
Common questions
Can I keep driving with a P0420?
In most cases yes, for a short period. The car will usually drive normally, but it will fail an emissions test and an unrepaired underlying fault will keep destroying converters.
Will replacing the oxygen sensor clear a P0420?
Only if the sensor is genuinely the fault. Replacing a sensor on a car with a failed converter changes nothing, which is why step 2 exists before any part is bought.
Last verified 2026-03-14 · Sources: SAE J2012 — Diagnostic Trouble Code Definitions and Failure Type Byte (SAE International)
This information is for general guidance and is not a substitute for professional diagnosis. Cost figures are estimates, not quotes.