P0420: Catalyst System Efficiency Below Threshold (Bank 1)
Updated Last reviewed September 15, 2026By CarFaultIQ Editorial Team
Verify with official recall databases, service records, a mechanic inspection, and local market data before relying on this summary.
At a glance
Quick diagnostic answer
The engine computer compares oxygen sensor readings before and after the catalytic converter. When the downstream sensor starts mirroring the upstream one, the converter is no longer storing and converting emissions efficiently, and P0420...
SeverityModerateUrgencyPlan a repair soonEvidenceMedium confidence
OBD code
P0420
System/category
Emissions & Catalyst
Severity
Moderate
Structured severity term
Urgency
Plan a repair soon
Structured rule output, needs verification
Can I drive?
Usually OK to drive short-term
The engine runs normally with this code alone, but unresolved upstream problems can destroy the converter over time.
Repair/cost band
$$$ · Major
Estimate band only — actual cost varies by region, parts availability and labor rates.
Vehicle applicability
Reported across virtually every make as vehicles age; high-mileage cars with mixture or oil-consumption history are the classic background.
Current applicability note is free text
Computed rule output may be more cautious than stored content and is never used to soften safety-critical risks.
Short answer
P0420 means the catalytic converter on bank 1 isn't cleaning exhaust as efficiently as it should. The car usually drives fine, but ignore it long enough and a failing converter — or whatever is poisoning it — gets expensive.
Ranked from published CarFaultIQ rule data for this page — not a remote diagnosis. Other causes are possible.
UrgencyPlan a repair soon
Can I keep driving?Drive with caution — limit trips; avoid heavy loads and long highway runs
Check this firstCheck the oil level and watch for blue exhaust smoke after long idling
Ask the mechanic to verifyVerify the oil consumption rate and inspect the catalyst before replacing parts
Do not ignore ifOil level drops quickly between checks or the light starts flashing
SeverityModerateDrive?Usually OK to drive short-termRepairProfessional
Key takeaways
The car usually drives fine, but ignore it long enough and a failing converter — or whatever is poisoning it — gets expensive.
Most common causes: aging or failed catalytic converter and lazy or failed downstream o2 sensor mimicking converter failure.
First diagnostic step: scan for companion codes first — misfires (P03xx) or lean codes (P0171) must be fixed before judging the converter.
Can I still drive with this?
Usually OK to drive short-term
The engine runs normally with this code alone, but unresolved upstream problems can destroy the converter over time.
SeverityModerate
Moderate
Diagnostic flow
What it meansThe engine computer compares oxygen sensor readings before and after the catalytic…
Check this firstScan for companion codes first — misfires (P03xx) or lean codes (P0171) must be fixed before judging the converter
Likely causeAging or failed catalytic converter
Can you drive?Usually OK to drive short-term
P0420 is a generic OBD-II code, which means it carries the same core definition on every make — what differs is which causes are most likely on your engine.
Work the diagnostic steps below in order. They are sequenced from cheapest-and-most-common to most expensive, which is exactly the order a fair shop should follow.
Structured DTC reference
What P0420 means
The catalytic converter is not cleaning the exhaust as well as it should
A trouble code identifies the circuit or monitored condition that failed its test. It does not, by itself, identify the failed part or prove the same cause on every vehicle.
Cause categories to discriminate
Worn-out catalytic converter
Blocked catalytic converter
Lazy oxygen sensor
These are structured diagnostic hypotheses, not a parts-replacement list. Vehicle context, symptoms, operating conditions, and test results determine which remain supported.
Severity and drivability
The code alone is not enough to assign one universal severity or “safe to drive” answer. Stop driving for loss of braking or steering, severe overheating, oil-pressure loss, a fuel leak or fire risk, or any rapidly worsening safety symptom.
The engine computer compares oxygen sensor readings before and after the catalytic converter. When the downstream sensor starts mirroring the upstream one, the converter is no longer storing and converting emissions efficiently, and P0420 sets.
Common symptoms
Check engine light on, often with no drivability change
Sulfur (rotten egg) smell in some cases
Failed emissions test
Reduced fuel economy when an underlying mixture fault exists
Likely causes
Aging or failed catalytic converter
Lazy or failed downstream O2 sensor mimicking converter failure
Exhaust leaks upstream of the sensors skewing readings
Engine misfires or oil/coolant burning poisoning the converter
Diagnosis steps
Scan for companion codes first — misfires (P03xx) or lean codes (P0171) must be fixed before judging the converter
Inspect the exhaust for leaks from the manifold back
Watch live O2 sensor data: a downstream sensor that switches like the upstream one points to the converter
Rule out oil or coolant consumption that poisons converters
Only then price converter replacement — and use a quality unit; cheap converters commonly re-set P0420
What a mechanic should check
Verify the converter is the victim, not the crime scene: misfires, leaks and oil burning all set P0420 and will kill a new converter too.
Vehicle applicability
Reported across virtually every make as vehicles age; high-mileage cars with mixture or oil-consumption history are the classic background.
Repair actions / cost band
$$$ · Major
Estimate band only — actual cost varies by region, parts availability and labor rates.
P0420 — vehicles with official records naming this code
P0420 is named in NHTSA-published manufacturer communications or investigations for 84 vehicle families, including:
A code named in a manufacturer communication means the document refers to it for those vehicles. It is not a diagnosis of any car, and it does not mean the code is common on that vehicle.
Frequently asked questions
Can I drive with P0420?
Usually yes in the short term if the engine runs smoothly — but book a diagnosis. A flashing check engine light or misfire changes the answer to no.
Will a fuel additive fix P0420?
Additives occasionally clear marginal cases temporarily, but they do not repair a worn converter or fix the upstream cause.
Why did P0420 come back after a new converter?
Usually because the original cause — a misfire, leak, sensor or oil burning — was never fixed, or a low-grade converter was fitted.
The Toyota Camry is commonly reported to experience higher-than-normal oil consumption linked to piston rings, often associated with the 2007–2011 model years.…
SeverityHigh→
Why these are related
Honda Accord: V6 Oil Consumption (VCM Years) (2008–2012)Long-term oil burning can degrade the catalyst and raise P0420.
Toyota Camry Piston Ring Oil BurningThis oil-burning fault can poison the catalyst and raise P0420.