P0420 on a 2018 Toyota Camry
- Vehicle
- 2018 Toyota Camry
- Engine
- A25A-FKS
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 is different on this vehicle
On the XV70 Camry the downstream oxygen sensor is the more common fault of the two, not the converter. The A25A-FKS runs a high compression ratio and a lean-burn cycle that keeps exhaust temperature lower than the 2AR-FE it replaced, so converters on this generation last noticeably longer while sensors age at the usual rate. Verify the sensor before quoting a converter.
Symptoms you may notice
What causes P0420
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.
On this vehicle: The rear sensor is the leading cause of P0420 on this generation. It is reachable from above and is a far cheaper repair than the converter.
Common misdiagnosis
Compare upstream and downstream sensor traces on a live-data graph before condemning the converter.
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.
On this vehicle: Genuine converter failure is uncommon on the XV70 before roughly 200,000 km unless a misfire was left unrepaired.
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.
On this vehicle: On the XV70 the front section of the exhaust is reached from above once the plastic engine cover is unclipped; there is no need to raise the car to inspect the manifold joint.
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.
Related codes
General information about P0420 →
Common questions
Does a P0420 on a 2018 Camry mean the converter has failed?
Usually not. On this generation the downstream oxygen sensor is the more common cause, and the converter is unlikely to be spent before roughly 200,000 km. Graph both sensors before buying any part.
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); 2018 Camry Repair Manual, engine and emission control section (Toyota Motor Corporation)
This information is for general guidance and is not a substitute for professional diagnosis. Cost figures are estimates, not quotes.