P0420 Catalytic converter efficiency below the computer's limit (Bank 1)

Catalyst efficiency below threshold, bank 1

Monitor

With a steady light and an engine that runs normally, this code reports reduced emissions cleanup rather than an immediate driving hazard; get it diagnosed rather than ignored, because the root cause can be upstream and can worsen.

It gets more serious if:

  • Stop: Stop if the check engine light starts flashing: that means an active misfire hot enough to damage the catalytic converter. Pull over where nothing flammable is under the car and let it cool.
  • Inspect soon: Inspect soon if the engine stalls, idles roughly or other engine codes are stored alongside it.

Source: how we rate severity

P0420 in short

P0420 means the computer's converter test found the bank 1 catalytic converter storing less oxygen than it should. The converter is often worn, but misfires, mixture problems, exhaust leaks and oxygen sensor faults must be ruled out first. With a steady light and normal running, short-term driving is usually fine; if the light flashes, stop.

Your car has an oxygen sensor before the catalytic converter and another after it. The front sensor swings between rich and lean as the computer trims the fuel. A healthy converter stores and releases oxygen, so the rear sensor stays fairly steady. Many engine computers also run an active test, briefly driving the mixture rich and then lean and timing how quickly the rear sensor follows.

When the rear signal follows the front one too closely or too quickly, the computer concludes the converter on bank 1 (the side with cylinder 1) has lost capacity and sets P0420. Emissions rules require this monitor so a converter that no longer cleans the exhaust is flagged.

The code reports a test result, not a failed part. A misfire, a mixture that is too rich or lean, an exhaust leak that lets air reach the sensors, or a lazy oxygen sensor can all fail the test on a converter that is still usable. That is why a good shop checks those first, and why a new converter can fail again if the root cause is left in place.

Most likely causes

These are general causes for this code across vehicles. None is specific to one make or model.

  1. Catalytic converter on bank 1 worn out or damaged

    Common general cause
    Why it ranks here
    The converter is the part this test is built to catch, and converters lose oxygen-storage capacity with age and heat. It ranks first only on the condition that the checks below come back clean; a shop should not condemn it on the code alone.
    What would change this
    Clean results on misfire, fuel trim, exhaust leaks and sensor response move this to the top; a rear sensor that copies the front sensor's switching, or visibly damaged or melted converter faces, confirms it.
    How to test the catalytic converter

  2. Exhaust leak near the oxygen sensors

    Common general cause
    Why it ranks here
    Even a small leak ahead of or between the sensors lets outside air reach them, which skews the readings the converter test relies on. OEM service information checks the exhaust joints before the converter.
    What would change this
    Ticking at cold start, soot at a joint or a smoke test that shows a leak confirms it; a sealed exhaust moves the sensors and converter up.

  3. Slow or biased oxygen sensor on bank 1 (front or rear)

    Common general cause
    Why it ranks here
    The computer judges the converter only through its oxygen sensors, so a lazy or biased sensor can fail the test on a good converter. Sensors also wear out with mileage.
    What would change this
    A graph showing a rear sensor that is stuck, flat or slow to react to rich and lean changes, or a sluggish front sensor, raises this; two sensors that respond crisply lowers it.
    How to test the oxygen (o2) sensor

  4. Misfire or a rich/lean mixture feeding the converter

    Common general cause
    Why it ranks here
    A converter can't be judged fairly while the engine sends it unburned fuel or the wrong mixture, and both conditions also shorten converter life. That is why misfire and fuel-trim problems are fixed first.
    What would change this
    Misfire counts or other misfire/fuel-trim codes raise this; fuel trims near zero with no misfire lowers it.
    How to test the spark plug

  5. Converter coated by burning oil or leaking coolant

    Possible — needs testing
    Why it ranks here
    Oil or coolant entering the combustion chamber leaves deposits in the exhaust that contaminate sensors and the converter. It matters because a new converter fails again if the engine keeps burning oil or coolant.
    What would change this
    Oily black or green, grainy deposits on the oxygen sensor tip, rising oil use or unexplained coolant loss raise this; clean sensor tips and stable fluid levels lower it.

  6. Engine computer needs a software update for the catalyst test

    Possible — needs testing
    Why it ranks here
    On some vehicles the maker revised the catalyst-monitor calibration. It is a quick check, but only applies where a bulletin or update exists for your VIN.
    What would change this
    A dealer or bulletin lookup confirming an update for your VIN raises this; no update available rules it out.

What to test first

Start at step 1. Each result tells you what it means and where to go next.

  1. Read every stored and pending code and the freeze-frame data before clearing anything

    Low riskTool: OBD-II scan tool

  2. With the engine fully warm, watch short- and long-term fuel trims and misfire counters at idle and at about 2,500 rpm

    Use cautionTool: scan tool with live data

    Warning:Keep hands, hair and loose clothing clear of the belts and fan while the engine runs, and never run the engine in a closed garage.

  3. Check the exhaust from the engine to just past the rear oxygen sensor for leaks: ticking at cold start, soot at joints, loose or cracked flanges. A shop can smoke-test it

    Use cautionTool: visual inspection; smoke machine (shop)

    Warning:Exhaust parts stay hot enough to burn skin long after the engine stops. Let the car cool for at least an hour, and never get under a car held up only by a jack.

  4. Graph the bank 1 front and rear oxygen sensors together on a warm engine, including a quick throttle snap and a closed-throttle coast-down

    Use cautionTool: scan tool with graphing

    Warning:Keep hands, hair and loose clothing clear of the belts and fan while the engine runs, and never run the engine in a closed garage.

    • IfThe rear sensor is stuck, flat or slow to follow the rich and lean changes, or the front sensor is sluggish

      It meansA sensor, not the converter, is the likely fault.

      Points to: Slow or biased oxygen sensor on bank 1 (front or rear)

    • IfBoth sensors respond crisply, but the rear sensor copies the front sensor's fast switching at steady speed

      It meansThe converter is no longer storing oxygen; confirm before replacing it.

      Go to step 5

  5. Have a shop check for a software update or bulletin for your VIN, inspect the converter for physical damage, and look at the sensor tips for oil or coolant deposits

    Best left to a shopTool: shop diagnosis

    Warning:Converter temperature checks and sensor removal involve a hot exhaust and a raised car; leave them to a shop with a lift. Exhaust parts stay hot enough to burn skin long after the engine stops. Let the car cool for at least an hour, and never get under a car held up only by a jack.

Symptoms

  • Steady check engine light with no change in how the car drives (the most common case)
  • Failed or refused emissions inspection
  • Engine stalls or idles roughly, or other engine codes are stored Gets more serious: stop and have it checked.
  • Check engine light flashing, or the engine shakes and stumbles under load Gets more serious: stop and have it checked.

Compare with: Car shaking with the check engine light on

How to fix

  1. Write down every stored and pending code and the freeze-frame data before clearing anything; clearing first erases evidence and resets the readiness monitors.

  2. With the engine cold, look along the exhaust between the engine and the rear oxygen sensor for loose, rusted-through or sooty joints.

    Warning:Only inspect a fully cooled exhaust, and never get under a car supported only by a jack.

  3. Ask a dealer whether a software update or bulletin applies to your VIN, and whether the converter is still inside the federal 8-year/80,000-mile emissions warranty.

Parts and cost context

  • Catalytic converter:Usually the most expensive part in this diagnosis. Confirm the cause first, check emissions-warranty coverage, and use a converter certified for your vehicle. Labor varies; get a quote.
  • Oxygen (O2) sensor:Much cheaper than a converter, but replace one only after a test shows it is slow, biased or failed.

Labor varies by vehicle and shop; get a quote before you approve a repair.

Compare used part prices on PartPriceGuide

On your vehicle

Saab

  • 2005 · 9-2X · 2.5i

    Supported by vehicle data

    Saab's service procedure for the 2005 9-2X 2.5i has the technician diagnose any other stored code first, then check the rear oxygen sensor wiring, then look for exhaust leaks or air being drawn in at three joints (cylinder head to front pipe, front pipe to front converter, front to rear converter), and only then inspect the front converter's faces for damage.

Ford

  • 2020–2022 · F-250 Super Duty

    Supported by vehicle data

    On listed 2020-2022 Super Duty trucks, Ford says the light can come on incorrectly with P0420 because of PCM software. Check for this update before any converter work.

  • TSB 26-2216Status unverified

    MIL on with P0420 and/or P0430 from PCM software

    2020–2022f-250-super-duty, f-350-super-duty, f-450-sd, f-550-sd · issued · NHTSA 11032694

    Ford says the light can come on incorrectly with P0420/P0430 on listed Super Duty trucks because of PCM software, and that the condition does not affect drivability or durability. Supersedes 25-2549.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

Dodge

  • 2019 · Journey

    Supported by vehicle data

    On the 2019 Journey, a PCM software update is the bulletin repair for P0420 or P0430, including a failed emissions test.

    NHTSA owner complaints mentioning P0420: 7. That shows what owners reported, not how often a cause sets the code.

  • 18-063-26Status unverified

    PCM software update for P0420/P0430

    2019journey · issued · NHTSA 11034359

    For a lit MIL with P0420 or P0430 (and possibly a failed emissions test), the bulletin reprograms the PCM with the latest software.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

  • P0430Catalytic converter efficiency below the computer's limit (Bank 2)

    Other bank Common general cause

    P0430 is the same converter test on the other bank. Seeing both together points to something both banks share (fuel, misfire across the engine, an exhaust leak before the pipes split, or age) rather than two separate failures.

  • P0300Random or multiple-cylinder misfire detected

    Primary / secondary Common general cause

    Likely primary: P0300. Misfire sends unburned fuel into the converter and can both fail and damage it. Fix the misfire first, then recheck P0420.

  • P0171Engine computer has reached its limit adding fuel on bank 1: the mixture is lean

    Primary / secondary Possible — needs testing

    Likely primary: P0171. A lean mixture on bank 1 can make the converter test fail and stresses the converter. Fix the lean condition first.

  • P0172Engine computer has reached its limit removing fuel on bank 1: the mixture is rich

    Primary / secondary Possible — needs testing

    Likely primary: P0172. A rich mixture overloads and overheats the converter and can fail its test. Fix the rich condition first, then recheck P0420.

  • P0137The downstream oxygen sensor on bank 1 is reporting a voltage that stays too low

    Primary / secondary Common general cause

    Likely primary: P0137. P0137 is a fault in the rear oxygen sensor or its wiring, the same sensor the converter test depends on. OEM service information has you fix other codes before diagnosing P0420.

  • P0138The downstream oxygen sensor on bank 1 is reporting a voltage that stays too high

    Primary / secondary Common general cause

    Likely primary: P0138. P0138 is a fault in the rear oxygen sensor or its wiring, the same sensor the converter test depends on. Fix it first, then see whether P0420 returns.

  • P0141The heater in the downstream oxygen sensor on bank 1 isn't working as the computer expects

    Primary / secondary Possible — needs testing

    Likely primary: P0141. P0141 means the rear sensor's heater has failed; a cold rear sensor responds poorly, which can upset the converter test. Repair the heater circuit first.

Seen together

All Catalytic converter (catalyst efficiency) codes

P0420 questions

Can I keep driving with P0420?

Usually yes, for a short time, if the light is steady and the car runs normally. Get it diagnosed soon so an upstream problem doesn't damage a new converter. If the light starts flashing, pull over where nothing flammable is under the car and let it cool; that is a misfire that can overheat the converter.

Will my car pass an emissions test with P0420?

Not while the check engine light is commanded on: under the federal OBD inspection procedure, a lit MIL with stored codes is a failure. After a repair, the car also needs enough driving for its readiness monitors to complete before a retest.

Will a new oxygen sensor fix P0420?

Only if a sensor is actually at fault. Graph the sensors first: a stuck or slow sensor is worth replacing; sensors that respond crisply while the rear one copies the front point to the converter instead.

Is the catalytic converter covered by warranty?

Federal law requires an 8-year/80,000-mile warranty on major emission parts such as the catalytic converter. Check your warranty booklet and mileage before paying for a converter.

How long after the repair will the light go off?

The converter test only runs under specific driving conditions, so it can take several trips. Once the fault is gone, the light normally goes out after three consecutive good drives, and the stored code clears itself after about 40 warm-up cycles.

Sources and review

  1. Final Regulation Order: 13 CCR §1968.2, Malfunction and Diagnostic System Requirements (OBD II) for 2004 and later passenger cars, light-duty trucks and medium-duty vehicles — California Air Resources Board
  2. P0420 Code: The Guide to Diagnosis and Fast Fixes — Identifix ,
  3. 2026 Honda Pilot Owner's Manual: Indicator Coming On/Blinking (Malfunction Indicator Lamp) — American Honda Motor Co.
  4. Saab 9-2X 2.5i (2005) WIS DTC P0420: Catalyst system efficiency below threshold (bank 1) — Saab Automobile (Saab WIS service information, hosted by Saab Club of North America)
  5. How to Fix P0430 Code: A Guide for Auto Mechanics — Identifix
  6. Saab 9-3 (2008) WIS DTC P0420 00 (ME9): Three-way catalytic converter malfunction — Saab Automobile (Saab WIS service information, hosted by Saab Club of North America)
  7. Bosch Oxygen Sensors trade brochure — Robert Bosch LLC ,
  8. OBD: Frequently Asked Questions (EPA420-F-02-014) — U.S. Environmental Protection Agency, Office of Transportation and Air Quality ,
  9. Bosch Oxygen Sensors consumer brochure — Robert Bosch LLC ,
  10. P0171 Code: Causes, Symptoms, and How to Fix It — Identifix ,
  11. Ford Motor Company bulletin TSB 26-2216: MIL on with P0420 and/or P0430 from PCM software — Ford Motor Company (filed with NHTSA) ,
  12. FCA US (Stellantis) bulletin 18-063-26: PCM software update for P0420/P0430 — FCA US (Stellantis) (filed with NHTSA) ,
  13. NHTSA ODI consumer complaints flat files — National Highway Traffic Safety Administration (NHTSA), Office of Defects Investigation
  14. 40 CFR §85.2222 Onboard diagnostic test procedures — U.S. Environmental Protection Agency (Electronic Code of Federal Regulations)
Last reviewed
Written by
Saurabh K.
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