Failed or under-performing catalytic converter (the most likely cause, per OBD-Codes.com). Damaged or failed oxygen sensor. Damaged or misconnected downstream O2 sensor wiring. Exhaust leak at the manifold, pipes, converter or muffler. Cylinder misfire. Leaking fuel injector or high fuel pressure. Faulty engine coolant temperature sensor. Retarded spark timing. Oil contamination. Leaded fuel used (unlikely today).

Repair exhaust leaks. Replace a faulty downstream O2 sensor or repair its wiring. Fix the underlying engine problem (misfire, fuel, coolant sensor). Replace the catalytic converter. OBD-Codes.com recommends an OEM unit, or a high-quality 50-state-legal one, because cheap aftermarket converters often bring the code back.

P0420🌫 Emissions / exhaustDrive with caution, fix soon

P0420: Catalyst System Efficiency Below Threshold (Bank 1) (Meaning, Causes, Fixes)

Q: What does P0420 mean?

P0420 means the engine computer thinks the catalytic converter on Bank 1 isn't cleaning the exhaust as well as it should. It compares the oxygen sensors before and after the converter; if their readings look too similar, the code sets. Diagnose before buying a converter: exhaust leaks, O2 sensors and engine problems can cause it too. The catalytic converter has an oxygen (O2) sensor in front of it and one behind it. With a warm engine in closed loop, the upstream sensor's signal should swing up and down while the downstream sensor stays fairly steady. When the two readings look too similar, the PCM decides the converter's efficiency is below its threshold and turns on the check engine light. Bank 1 is the side of the engine with cylinder #1. OBD-Codes.com notes converters aren't normally a wear item, so if one has failed, something else probably caused it. RepairPal adds that converters usually fail because something else, like an engine misfire or an internal oil or coolant leak, wasn't repaired.

Q: Can I keep driving with P0420?

Drive with caution, fix soon. Usually you can keep driving short-term: OBD-Codes.com says you likely won't notice drivability problems. But don't ignore it. If the catalyst is breaking up it can restrict the exhaust (reduced power), and a misfire or fuel problem that's damaging the converter should be fixed first. If the check engine light is flashing, stop driving soon. Note that sources differ: RepairPal rates P0420 'Urgent', while OBD-Codes.com notes most drivers won't notice drivability problems. We treat it as 'fix soon', not as an emergency.

Reviewed 2026-10-21 · Based on 3 cited source(s)

What P0420 means

The catalytic converter has an oxygen (O2) sensor in front of it and one behind it. With a warm engine in closed loop, the upstream sensor's signal should swing up and down while the downstream sensor stays fairly steady. When the two readings look too similar, the PCM decides the converter's efficiency is below its threshold and turns on the check engine light. Bank 1 is the side of the engine with cylinder #1. OBD-Codes.com notes converters aren't normally a wear item, so if one has failed, something else probably caused it. RepairPal adds that converters usually fail because something else, like an engine misfire or an internal oil or coolant leak, wasn't repaired.

Severity: can I keep driving?

Drive with caution, fix soon

Usually you can keep driving short-term: OBD-Codes.com says you likely won't notice drivability problems. But don't ignore it. If the catalyst is breaking up it can restrict the exhaust (reduced power), and a misfire or fuel problem that's damaging the converter should be fixed first. If the check engine light is flashing, stop driving soon. Note that sources differ: RepairPal rates P0420 'Urgent', while OBD-Codes.com notes most drivers won't notice drivability problems. We treat it as 'fix soon', not as an emergency.

Common symptoms

Check engine light on (often the only symptom)
Usually no noticeable drivability change
Reduced power if the converter's internals have broken up and restrict the exhaust

Likely causes

Roughly most to least common, based on the sources below. Your vehicle may differ.

Failed or under-performing catalytic converter (the most likely cause, per OBD-Codes.com)
Damaged or failed oxygen sensor
Damaged or misconnected downstream O2 sensor wiring
Exhaust leak at the manifold, pipes, converter or muffler
Cylinder misfire
Leaking fuel injector or high fuel pressure
Faulty engine coolant temperature sensor
Retarded spark timing
Oil contamination
Leaded fuel used (unlikely today)

Diagnostic steps you can do

⚠ Before you start

Work on a cool engine, parked on level ground with the parking brake set. Never work under a car supported only by a jack.

Read all stored codes with a scanner. Fix any misfire (P0300–P0308) or fuel trim (P0171/P0174) codes first; they can damage the converter.
Check the exhaust for leaks at the manifold, pipes and converter (listen for ticking or puffing at cold start, look for soot marks).
Inspect the downstream O2 sensor and its wiring and connector for damage.
With a scanner that shows live data, compare the upstream and downstream O2 sensor readings on a warm engine. The upstream should fluctuate; the downstream should be fairly steady.
With an infrared thermometer on a fully warmed engine, compare the exhaust temperature just before and after the converter. OBD-Codes.com says the outlet should be about 100°F hotter.

Typical fixes

Repair exhaust leaks
Replace a faulty downstream O2 sensor or repair its wiring
Fix the underlying engine problem (misfire, fuel, coolant sensor)
Replace the catalytic converter. OBD-Codes.com recommends an OEM unit, or a high-quality 50-state-legal one, because cheap aftermarket converters often bring the code back.

Repair costs vary widely by vehicle, region, parts brand and shop, so we don't quote prices. Get a written estimate after diagnosis.

🛠 When to call a pro

You need an oscilloscope or advanced scan tool to confirm O2 sensor operation
The converter needs replacing (welding, emissions legality)
Your car may still be under the manufacturer's emissions-parts warranty; ask a dealer before paying for a converter

Where to buy

Plain search links. Always match parts to your exact year, make, model and engine.

OBD2 scanners on Amazon Parts at Advance Auto Parts catalog at RockAuto

FAQs

Should I just replace the O2 sensor?

OBD-Codes.com calls that probably the biggest mistake owners make with P0420. Diagnose properly first so you don't replace parts unnecessarily.

Is a converter covered by warranty?

RepairPal says catalytic converters are covered by a federally mandated 8-year/80,000-mile manufacturer warranty, and some makers (it names Volkswagen, Audi, Dodge and Toyota) have extended coverage on many vehicles. The root cause of the failure (such as a misfire) usually isn't covered. Ask a dealer service writer to check your VIN.

What's the difference between P0420 and P0430?

P0420 is Bank 1; P0430 is the same test on Bank 2 (V6/V8 engines).

Does the brand of replacement converter matter?

Both sources say it can. RepairPal notes most Toyotas usually set P0420 again unless an OEM catalyst is used, and OBD-Codes.com recommends OEM or a high-quality 50-state-legal unit.

Sources

Last reviewed 2026-10-21. Code definitions and repair steps can vary by manufacturer. Confirm with your vehicle's service information.