P0420 and P0430 together: both catalytic converters below the limit
Severity: Monitor
With a steady light and an engine that runs normally, two converter-efficiency codes report reduced emissions cleanup rather than an immediate driving hazard, but the cause is often upstream and can keep damaging new converters if it is not found.
It gets more serious if:
- Inspect soon: Inspect soon if the engine idles roughly, lacks power or other engine codes are stored with them, especially misfire or fuel-trim codes: those are likely causes, and a restricted converter can cut power.
- Stop: Stop if the check engine light starts flashing: that is misfire bad enough to overheat and damage the converters. Pull over where nothing flammable is under the car and let it cool.
The two codes
Why they show up together
P0420 (bank 1) and P0430 (bank 2) are the same efficiency test run on each side of a V-engine. The computer compares the oxygen sensor before each converter with the one after it; a working converter evens out the exhaust so the rear sensor stays fairly steady.
Converter makers say converters rarely fail on their own and that the root cause should be found before replacement: misfire overheats them, rich running overloads them, and burning oil or coolant coats them. Each of those can reach both banks, so they are worth ruling out first, though two separate one-bank faults remain possible.
Looking upstream first is our reasoning from those sources, not a guarantee. Replacing both converters without finding the cause risks the new ones failing the same way.
Shared causes
These are general causes for this code across vehicles. None is specific to one make or model.
Cause 1: Past or ongoing misfire overheating the converters
Common general cause- Why it ranks here
- Unburned fuel from misfire burns inside the converter and overheats it. Walker names misfire as the main cause of converter meltdown and tells technicians to resolve P0300-series codes before diagnosing a converter code; the regulation requires the light to flash when misfire is at a catalyst-damaging level.
- What would change this
- Raised if misfire codes are stored or were recently repaired, or misfire counters show activity. Lowered if there is no misfire history and counters are clean.
Cause 2: Rich running, oil burning or a coolant leak contaminating both converters
Common general cause- Why it ranks here
- Converter makers and Motor Age list rich mixtures, oil blow-by and coolant entering the exhaust as upstream causes that coat or overheat the converter. Each can affect both banks.
- What would change this
- Raised if fuel trims are strongly negative, a rich code is stored, or the engine uses oil or coolant. Lowered if trims are normal and no oil or coolant is being consumed.
Cause 3: Slow downstream oxygen sensors or an exhaust leak near them
Possible — needs testing- Why it ranks here
- The test relies on the downstream sensors. Walker notes that an exhaust leak near a sensor and other conditions can fail the test with a good converter, and that sluggish sensors are a cause to rule out. This explains both codes only if both sides are affected, so it ranks below whole-engine causes.
- What would change this
- Raised if a leak is found before the rear sensors or the sensors respond slowly to a forced rich or lean change. Lowered if the exhaust is sealed and sensor response is normal.
Diagnose in this order
- Fix every other stored code first, especially misfire codes (P0300 to P0306), fuel-trim codes and oxygen sensor codes. The converter test is only valid when the engine and sensors are working properly.
- Check for exhaust leaks ahead of or near the downstream oxygen sensors on both sides. Outside air leaking in can make good converters fail the test.
- With a scan tool, watch the upstream and downstream oxygen sensors on both banks with the engine fully warm. Downstream sensors that switch nearly as fast as the upstream ones support worn converters; slow or lazy sensors point to the sensors instead.
- Look for signs of oil or coolant being burned (blue or white smoke, oil or coolant use) and for a history of misfire or rich running, because those damage converters on both banks.
- Only after the upstream causes are ruled out, have the converters assessed for replacement. Converter work involves very hot exhaust parts and is a job for a shop.
Sources and review
- What Causes a Catalytic Converter to Fail
- Diagnosing Catalytic Converter Problems
- How to Fix P0420 Emissions Code
- Catalytic Converter FAQ
- Problems With Bad Catalytic Converters
- What's really wrong when a catalytic converter 'fails' (Mike Mavrigian)
- Oxygen Sensors (technical PDF)
- Final Regulation Order: 13 CCR §1968.2, Malfunction and Diagnostic System Requirements (OBD II), 2004 and subsequent model-year passenger cars, light-duty trucks and medium-duty vehicles (2015 amendments)
- On-Board Diagnostic II (OBD II) Systems Fact Sheet
- 2023 Integra Owner's Manual: Indicator, Coming On/Blinking
- OBD consumer brochure: Do you drive a 1996 or newer car or light truck? (EPA420-F-02-016)
- Last reviewed
- Written by
- Saurabh K.
- Editorial review
- Pending
- Technician review
- Technician review pending
This page explains what a code usually means; it is not a substitute for a professional inspection. How we source and review · Report an error