P0430
- Powertrain
- Generic (SAE)
- Emissions
Diagnose soon
Driveability is unaffected and nothing mechanical is at risk, but it is an automatic OBD emissions-test failure, and whatever degraded the converter will keep working on a replacement if it is not found.
- Safe to drive
- Safe to keep driving if the engine runs smoothly. Diagnose before your next emissions test, and before any underlying cause spreads.
Catalyst System Efficiency Below Threshold (Bank 2)
The computer compared exhaust gas before and after the bank 2 catalytic converter and found it no longer storing and releasing oxygen the way a healthy converter does. That is a measurement of converter performance, not proof the converter has failed.
The monitor logic is identical to P0420 but applied to bank 2 — the bank that does not contain cylinder 1. Only engines with two cylinder banks and separate exhaust paths can set this code, so it appears on V6, V8 and flat engines. A healthy three-way catalyst buffers oxygen, so the downstream sensor holds a comparatively steady voltage while the upstream sensor switches rapidly. As oxygen storage falls, the downstream trace begins to mirror the upstream one and the PCM's efficiency score crosses its threshold.
How the car detected itNon-continuous monitor requiring a warm engine, closed loop, steady cruise and intake air temperature above a minimum; some calibrations also require a minimum fuel level so that fuel starvation cannot corrupt the run, but there is no fuel-level window as there is for EVAP. It typically fails on two consecutive trips before the MIL lights. Bank 2 has its own upstream sensor, downstream sensor and converter, so a bank 2 failure with a normal bank 1 points at something specific to that side. P0420 and P0430 stored together usually mean shared age, or a shared upstream cause such as a rich condition, oil consumption, or coolant entering the exhaust.
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 if the engine runs smoothly. Diagnose before your next emissions test, and before any underlying cause spreads.
- Urgency
- Diagnose soon
- Will it pass emissions
- No
- If ignored
- The vehicle keeps driving but fails OBD emissions inspection. If an engine fault degraded the converter, that fault continues. On engines where both banks are the same age, the other converter is usually not far behind, so a single-converter repair may be followed by P0420 within a year.
- 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 | — |
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.
Which physical side of the engine is bank 2 on this specific vehicle
Bank 2 is the bank without cylinder 1, and its physical location varies by manufacturer and layout. On transverse V6 engines it can be the firewall side. Getting this wrong means testing the healthy half of the exhaust.
Whether P0420 is also stored
Both banks failing together points to shared age or a shared upstream cause — rich mixture, oil consumption, coolant leak — rather than one dead converter.
Fuel trims for bank 2 specifically
A rich bank 2 consumes that converter's stored oxygen and leaves its downstream sensor pegged high; a persistently lean bank saturates storage and pegs it low. Either produces a genuine efficiency failure with a healthy catalyst. Trims beyond roughly plus or minus 10% need explaining before anything is replaced.
Exhaust leaks on the bank 2 pipe, especially near the downstream sensor bung
Air drawn in through a leak makes that sensor read lean and imitates a failed converter. Cheapest item on the list, and a real cause.
Emissions warranty coverage and open technical service bulletins
The federal catalyst warranty runs 8 years or 80,000 miles on most US-market vehicles, and several manufacturers have issued PCM calibration updates specifically for catalyst-monitor false failures.
Identify bank 2 physically and map the live-data sensor names to real locations
Tools Service information, scan tool live data
Confirm from service information which bank contains cylinder 1, then confirm which sensor in live data corresponds to which physical sensor. Do this before touching a wrench — it is the most common wasted hour on this code.
ExpectedAn unambiguous match between the sensor names on the scan tool and the parts on the vehicle.
Check bank 2 fuel trims warm at idle and cruise
Tools Scan tool with live data
Read short-term and long-term trim for bank 2 with the engine fully warm and in closed loop, at idle and at a steady 2000 to 2500 RPM. Compare against bank 1 on the same drive.
ExpectedCombined trim within roughly plus or minus 10%, and comparable to bank 1. A mixture off in either direction must be corrected before judging the converter.
Pressure or smoke test the bank 2 exhaust
Tools Exhaust smoke machine or low-pressure exhaust tester
Check for leaks from the cylinder head all the way past the downstream sensor, including manifold gaskets, flex pipe and the sensor bungs. Leaks that only open when hot are common on flex sections.
ExpectedNo escaping smoke and no audible leak anywhere on that bank.
Graph bank 2 upstream and downstream sensors at steady cruise, alongside bank 1
Tools Scan tool with multi-PID graphing, or lab scope
Warm, closed loop, holding a steady 2000 to 2500 RPM or 40 to 50 mph, graph all four sensors. The side-by-side comparison of bank 2's downstream behaviour against bank 1's is the most useful evidence available on this code, because it gives you a reference from the same engine on the same drive.
ExpectedBank 2's downstream trace should be as flat and slow as bank 1's. A downstream trace copying its own upstream sensor's swings indicates spent oxygen storage.
Verify the bank 2 downstream sensor's own response
Tools Scan tool with live data or lab scope
Force rich with a snap throttle, then force lean with a closed-throttle deceleration that triggers fuel cut, and watch the sensor react. A drifted or lazy rear sensor is a fraction of the cost of a converter and produces the same code.
ExpectedRises 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 bank 2 uses a wideband downstream sensor, judge response speed and range against the manufacturer's live-data description rather than a switching voltage. Sluggish or narrow response indicts the sensor.
Look for what contaminated the converter
Tools Visual inspection, cooling system pressure tester
Oil consumption, coolant loss, over-fueling from a leaking injector, and non-sensor-safe silicone sealants all poison catalysts. Check oil and coolant levels, look for blue or white exhaust smoke, and review recent gasket work.
ExpectedNo oil or coolant consumption, no visible smoke, no history of running with a misfire.
Check bank 2 for exhaust restriction
Tools Exhaust backpressure gauge or vacuum gauge
A converter can be plugged as well as spent. Measure backpressure through the bank 2 upstream sensor port under sustained load, or watch for power loss and falling manifold vacuum at high RPM.
ExpectedBackpressure stays low and stable under load. The threshold is vehicle-specific; a steadily climbing reading indicates restriction.
Repair, then complete a drive cycle and confirm the monitor
Tools Scan tool with readiness monitor status
Clear codes and drive until the catalyst monitor reports complete. This is a non-continuous monitor and may take several trips, so absence of the light within a day proves nothing.
ExpectedCatalyst monitor complete and passing with no return of P0430.
| Repair | Only after you confirm | Typical cost | Who |
|---|---|---|---|
| Repair exhaust leak on bank 2 upstream of, or at, the downstream sensor | Leak located with the exhaust smoke or pressure tested | $100–$500 | DIY |
| Replace bank 2 downstream oxygen sensor | Sensor slow, biased, or failing to swing fully under forced rich and lean conditions | $150–$400 | DIY |
| Correct a rich condition, oil consumption, or coolant intrusion feeding bank 2 | Fuel trims outside plus or minus 10%, measurable oil consumption, or coolant loss confirmed before converter work | $200–$2,500 | Hard |
| Replace the bank 2 catalytic converter | Trims normal, exhaust verified leak-free, downstream sensor verified responsive, and its 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 |
Bank 2 converters are frequently the harder ones to reach. On transverse V6 engines the rear-bank converter can sit against the firewall and add an hour or more of labor compared with the front. Installed prices commonly run $700 to $2,600, and can pass $3,000 for a CARB-compliant, OEM, or manifold-integrated part. CARB-state vehicles are restricted in which converters may legally be fitted. Check the 8 year / 80,000 mile federal emissions warranty before paying.
- Working on the wrong bank. Bank 2 is not simply the passenger side on every vehicle.
- Replacing the converter before verifying fuel trims, exhaust integrity, and the downstream sensor's response.
- Replacing one converter when both banks are the same age, then meeting P0420 a few months later. Check the bank 1 sensor pattern while the tools are already connected.
- Fitting a low-grade universal converter that lacks the precious-metal loading to satisfy the monitor.
- Clearing the code before an emissions test, which resets readiness monitors and fails the test for incompleteness instead.
- Treating a sulfur smell as proof the converter is dead. It usually indicates a rich condition feeding it, which is the thing to fix.
- 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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