P0420
- Powertrain
- Generic (SAE)
- Emissions
Diagnose soon
Nothing mechanical is at risk and the vehicle drives normally, but this is an automatic OBD emissions-test failure, and the condition that degraded the converter is often an engine fault that will keep destroying converters.
- Safe to drive
- Safe to keep driving in most cases if the engine runs smoothly. Get it diagnosed before it fails inspection or the real cause spreads.
Catalyst System Efficiency Below Threshold (Bank 1)
The computer compared the exhaust gas before and after the bank 1 catalytic converter and concluded the converter is no longer cleaning exhaust as well as it should. That is a measurement of performance, not proof the converter has failed.
In closed loop, a healthy three-way catalyst stores and releases oxygen, so the downstream oxygen sensor holds a fairly steady voltage while the upstream sensor swings rich to lean several times a second. As the catalyst's oxygen storage capacity falls, the downstream signal begins to copy the upstream signal. The PCM scores that similarity — switch-count ratio, amplitude ratio, or a commanded rich/lean oxygen-storage test depending on the manufacturer — and sets P0420 when the score stays past its threshold. Bank 1 is the bank containing cylinder 1.
How the car detected itThe catalyst monitor is non-continuous. It runs only after enable conditions are met: engine at operating temperature, closed loop, intake air temperature above a minimum, and a reasonably steady cruise held long enough for the monitor to sample. Some calibrations also require a minimum fuel level so that fuel starvation cannot corrupt the run, but there is no fuel-level window like the EVAP monitor's. During that window the PCM logs downstream oxygen sensor activity relative to upstream. High similarity between the two means low oxygen storage, which the PCM scores as reduced conversion efficiency. Most calibrations require the monitor to fail on two consecutive trips before the MIL lights, so a single bad tank of fuel or one rich event rarely triggers it.
A trouble code records what a control module measured. It does not identify which part failed. Test before replacing anything.
- Safe to drive
- Safe to keep driving in most cases if the engine runs smoothly. Get it diagnosed before it fails inspection or the real cause spreads.
- Urgency
- Diagnose soon
- Will it pass emissions
- No
- If ignored
- The car keeps driving but will fail an OBD emissions test. If a misfire, oil burning or rich condition caused the damage, that fault continues working on any replacement converter. Fuel economy usually drifts down slightly, and the illuminated MIL masks any new code that appears behind it.
- Check engine light
- Solid
- Clearing the light
- A completed drive cycle is normally required before the monitor re-runs and the light can clear.
- Failed emissions inspection
- Possibly slightly increased fuel consumption
- Sulfur (rotten egg) smell from exhaust under load
Many vehicles show no symptom at all beyond the warning light. The absence of a symptom does not mean the fault is not real.
These are the causes this code can have. For the order to work through them in, see what to check first.
| # | Cause | How often | Component |
|---|---|---|---|
| 1 | Catalyst aged or contaminated | Very common | — |
| 2 | Downstream oxygen sensor drift | Common | — |
| 3 | Exhaust leak before catalyst | Occasional | — |
| 4 | Long-term misfire damage | Occasional | — |
Ordering reflects how often each cause is responsible in general, not a probability for your vehicle. Confirm by testing.
In order. Each of these is cheaper or faster than what follows it, and each one can make the rest unnecessary.
Any other stored codes, especially misfire, fuel trim, or oxygen sensor codes
P0420 is frequently downstream of another fault. A lean, rich, or misfiring engine cooks converters. Fixing the converter without fixing the cause repeats the failure.
Freeze frame data from the moment the code set
It records engine load, RPM and coolant temperature during the failed monitor run, and shows whether fuel trims were already out of range at that moment.
Exhaust leaks between the engine and the converter, and around the downstream sensor bung
Air drawn in through a leak makes the rear sensor read lean and imitates a failed converter. It is the cheapest item on the list and a genuine cause.
Long-term fuel trim at idle and at steady cruise
Trims beyond roughly plus or minus 10% mean the mixture is off. A rich mixture consumes the converter's stored oxygen and leaves the downstream sensor pegged high; a persistently lean mixture saturates that storage and leaves it pegged low. Either extreme can fail the efficiency test with a perfectly healthy catalyst, so the mixture must be corrected before the converter is judged.
Emissions warranty coverage and open technical service bulletins
The federal catalyst warranty is 8 years or 80,000 miles on most US-market vehicles, and several manufacturers have released PCM calibration updates for catalyst-monitor false failures. A free repair beats a diagnosis.
Scan for all codes and capture freeze frame before clearing anything
Tools OBD-II scan tool capable of reading freeze frame
Pull every module's codes, not just powertrain. Save freeze frame. If misfire, lean, rich, or oxygen sensor heater codes are present, those are diagnosed and repaired first — P0420 is often their consequence.
ExpectedP0420 alone, with no misfire, fuel trim or sensor codes. Anything else present redirects the diagnosis.
Verify fuel trims are in range
Tools Scan tool with live data
Warm the engine fully into closed loop. Read short-term and long-term fuel trim for bank 1 at idle, at a steady 2000 to 2500 RPM, and under light load. Persistent positive or negative correction means the mixture is wrong and the converter is being asked to clean exhaust it was never designed for.
ExpectedCombined short and long-term trim within roughly plus or minus 10% at both idle and cruise.
Test for exhaust leaks upstream of the downstream sensor
Tools Exhaust smoke machine or low-pressure exhaust tester, mechanic's stethoscope
Pressurize or smoke the exhaust from the tailpipe with the system cool, or listen with a stethoscope at manifold joints, flex pipe, gaskets and sensor bungs while the engine idles. Small leaks that only open when hot are common on flex pipes.
ExpectedNo escaping smoke or audible leak anywhere between the cylinder head and the downstream sensor.
Graph both bank 1 oxygen sensors at steady cruise
Tools Scan tool with graphing, or a lab scope
With the engine warm and in closed loop, hold a steady 2000 to 2500 RPM or a steady 40 to 50 mph cruise and graph upstream and downstream sensor signals together. A healthy converter keeps the downstream trace comparatively flat and slow while the upstream trace cycles rapidly. A downstream trace that mirrors the upstream swings shows spent oxygen storage.
ExpectedDownstream trace flat and slow-moving. Note that wideband air-fuel-ratio upstream sensors report differently from conventional narrow-band sensors — interpret against the manufacturer's live data description.
Verify the downstream sensor itself before condemning the converter
Tools Scan tool with live data or a lab scope
Force the mixture rich with a snap throttle, then force it lean with a closed-throttle deceleration that triggers fuel cut, and watch the rear sensor respond. A drifted or sluggish rear sensor produces a genuine P0420 with a healthy converter, and it is a fraction of the cost.
ExpectedRear sensor rises above roughly 0.8V under enrichment and falls below roughly 0.2V on fuel cut, responding within about a second. These figures apply to a conventional narrow-band downstream sensor; where the vehicle uses a wideband downstream sensor, judge response speed and range against the manufacturer's live-data description rather than a switching voltage. Slow, stuck, or narrow-range response indicts the sensor.
Look for what poisoned or overheated the converter
Tools Visual inspection, cooling system pressure tester, oil consumption check
Check for oil consumption, coolant loss, and over-fueling. Blue exhaust smoke means oil, white sweet-smelling smoke means coolant, a strong fuel smell means over-fueling. Silicone sealants that are not sensor-safe and certain fuel additives also poison catalysts. A history of running with a misfire is the most common thermal killer.
ExpectedNo oil or coolant consumption, no smoke, no history of driving with a misfire.
Check for exhaust restriction
Tools Exhaust backpressure gauge or vacuum gauge
A converter can be physically plugged as well as chemically spent, and the symptoms differ — a restricted converter causes power loss under load. Measure backpressure through the upstream sensor port under a sustained load, or watch manifold vacuum fall off at high RPM.
ExpectedBackpressure stays low and stable under sustained load. The absolute threshold is vehicle-specific; a reading that climbs and keeps climbing indicates restriction.
Repair, clear, and confirm with a completed drive cycle
Tools Scan tool with readiness monitor status
After any repair, clear codes and drive until the catalyst monitor reports complete. This code cannot be confirmed fixed by absence of the light alone — the monitor is non-continuous and may take several trips to run.
ExpectedCatalyst monitor status shows complete and passing, with no return of P0420.
| Repair | Only after you confirm | Typical cost | Who |
|---|---|---|---|
| Repair exhaust leak upstream of or at the downstream oxygen sensor | Leak located by smoke or pressure test between the cylinder head and the rear sensor | $100–$500 | DIY |
| Replace bank 1 downstream oxygen sensor | Rear sensor slow, biased, or failing to swing fully under forced rich and lean conditions | $150–$400 | DIY |
| Correct the underlying fuel, oil consumption, or coolant intrusion fault | Fuel trims outside plus or minus 10%, measurable oil consumption, or coolant loss confirmed before any converter work | $200–$2,500 | Hard |
| Replace the bank 1 catalytic converter | Fuel trims normal, exhaust verified leak-free, rear sensor verified responsive, and the downstream trace still mirrors upstream at steady cruise | $700–$3,200 | Hard |
| Apply manufacturer PCM calibration update for the catalyst monitor | A technical service bulletin covering this vehicle's year, engine and symptom, AND the PCM's current calibration ID read with a scan tool confirmed to be older than the calibration the bulletin specifies | $100–$250 | Shop |
Converter cost varies enormously. A direct-fit aftermarket unit on a common four-cylinder can be under $800 installed, while a CARB-compliant or OEM converter, or one integrated into the exhaust manifold, can pass $3,000. California and states following CARB rules restrict which converters may legally be fitted, and a non-compliant part will fail inspection even if it works. Check the 8 year / 80,000 mile federal emissions warranty before paying for anything.
- Replacing the converter first. It is the most expensive part in the loop and is often the healthiest thing on the list.
- Ignoring fuel trims. A rich condition destroys converters, and a new one fails the same way within months.
- Assuming the rear oxygen sensor is always the fix. It is a real cause but a minority one, and swapping it blind wastes a good sensor.
- Fitting a cheap universal converter on an OBD-II vehicle. Many carry too little precious metal to satisfy the monitor and set P0420 again within a few thousand miles.
- Treating catalyst cleaner as a repair. It occasionally clears light sulfur contamination; it does not rebuild a thermally damaged substrate.
- Clearing the code just before an emissions test. Clearing resets readiness monitors, and the test then fails for incomplete monitors instead.
- Catalytic Converter
- Oxygen Sensor Upstream
- Oxygen Sensor Downstream
- Definition
- Generic code set (SAE J2012 / ISO 15031-6) via the OBDexopen database, dedicated to the public domain under CC0-1.0.
- Diagnostic guidance
- Written and reviewed in-house. See our editorial policy.
- Review status
- Reviewed
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