Emission controls · Powertrain · generic (SAE-defined)
P0420: Catalyst System Efficiency Below Threshold (Bank 1)
P0420 means the engine computer has determined that the catalytic converter on Bank 1 (the side with cylinder 1) is no longer cleaning exhaust as effectively as required.
Standard title per SAE J2012 / ISO 15031-6 · Updated 2026-11-07 · By CarWhere Research
How P0420 sets
The powertrain control module (PCM) compares the upstream oxygen sensor (Bank 1 Sensor 1), which switches rapidly as it reads raw exhaust, with the downstream sensor (Bank 1 Sensor 2) after the converter. A healthy converter stores oxygen and smooths the downstream signal into a fairly steady reading. When the downstream signal starts copying the upstream switching, the PCM infers reduced oxygen storage and sets P0420 after the test fails under specific warm, steady-driving conditions. The exact test and threshold are manufacturer-specific.
Common causes of P0420
Roughly most likely first; the actual cause depends on the vehicle and needs testing to confirm.
- Worn-out or damaged catalytic converter
- Converter damaged by an earlier misfire, rich running, oil burning, or coolant leak
- Exhaust leak near the oxygen sensors
- Faulty or slow downstream oxygen sensor (Bank 1 Sensor 2)
- Lazy upstream air-fuel or oxygen sensor skewing fuel control
- Engine running rich from leaking injectors or high fuel pressure
- Low-quality aftermarket converter that does not meet the vehicle's requirements
P0420 symptoms
- Check engine light on
- Often no drivability symptoms at all
- Rotten-egg or sulfur smell from the exhaust
- Reduced power or poor fuel economy if the converter is clogged
- Failed emissions inspection
How P0420 is diagnosed
- Scan for all codes and freeze frame; fix any misfire, fuel trim, oxygen sensor, or coolant temperature codes before condemning the converter
- Check for exhaust leaks ahead of and around the downstream oxygen sensor, including cracked manifolds and loose flanges
- Review fuel trims and look for signs of oil consumption or coolant loss that could have poisoned the converter
- Graph upstream and downstream oxygen sensor data at a steady warm cruise; a downstream signal that mirrors the upstream switching points to low converter efficiency
- Test the downstream sensor response by snapping the throttle or adding propane; a sluggish sensor can mimic a bad converter
- Check for converter restriction with a back-pressure or vacuum test if power loss is reported
- Check for updated PCM software, since some manufacturers revised converter monitor calibrations
Typical fixes for P0420
- Repair the root cause first: misfires, oil burning, coolant leaks, or rich running
- Repair exhaust leaks upstream of or near the downstream oxygen sensor
- Replace a faulty downstream or upstream oxygen sensor
- Replace the catalytic converter with a unit that meets federal or California requirements for the vehicle
- Apply a manufacturer software update if one exists
Replace parts only after testing confirms the cause — a code names the circuit or condition that failed a check, not necessarily the faulty part. We don’t publish per-repair prices; for what households actually spend on upkeep, see car repair costs.
Manufacturer notes
One of the most common codes across nearly every make, especially on higher-mileage vehicles. Some manufacturers have issued software updates that revise the converter monitor; check factory service information.
Related codes & warning lights
P0420 FAQ
What does code P0420 mean?
P0420 means the engine computer has determined that the catalytic converter on Bank 1 (the side with cylinder 1) is no longer cleaning exhaust as effectively as required. The standard definition is "Catalyst System Efficiency Below Threshold (Bank 1)."
Can I drive with a P0420 code?
In most cases yes, as long as the engine runs smoothly and the check engine light is steady rather than flashing. If you notice misfires, a flashing light, loss of power, or a sulfur smell, limit driving, because the root cause may be damaging the converter or causing a restriction.
Will replacing the catalytic converter fix P0420 for good?
Only if the converter was the sole problem. Converters usually fail because something upstream, like misfires, oil burning, a coolant leak, or rich running, overheated or contaminated them. If that cause is not fixed, a new converter can fail the same way.
Can a bad oxygen sensor cause P0420?
Yes. A slow or failing downstream oxygen sensor, or an exhaust leak near it that lets in outside air, can make a working converter look inefficient. These should be ruled out before replacing the converter.
Do catalytic converter cleaner additives fix P0420?
They rarely provide a lasting fix. Fuel additives cannot restore a converter that is physically damaged or chemically poisoned, though they will not usually cause harm.
Keep researching
More emission controls codes
Sources
- SAE J2012 — Diagnostic Trouble Code Definitions (standard code titles)
- ISO 15031-6 — Road vehicles: communication between vehicle and external equipment for emissions-related diagnostics, Part 6: Diagnostic trouble code definitions
- U.S. EPA — Vehicle Emissions Inspection and Maintenance (I/M) programs
- U.S. EPA, 40 CFR 86.1806-17 — Federal onboard diagnostics requirements for light-duty vehicles
- California Air Resources Board — On-Board Diagnostic (OBD) Program
General information written by CarWhere Research, not a diagnosis of your vehicle. Manufacturers can add their own sub-definitions and test conditions; confirm with the factory service information for your year, make and model. Before paying for a repair, check NHTSA for open recalls and your warranty booklet: federal law requires an emissions warranty on major emission-control parts such as the catalytic converter and engine computer.