P0421 means the ECU has detected that the warm-up catalytic converter on Bank 1 is not working as efficiently as it should during the early stages of engine operation. In simple terms, the catalytic converter is taking too long to reach operating temperature or is no longer cleaning exhaust gases effectively. This can lead to increased emissions, rougher cold starts, and reduced fuel economy. In this guide, you’ll learn exactly what P0421 means, what causes it, how to diagnose it, and the repairs that typically resolve the issue.
What Does P0421 Mean?
P0421 sets when the ECU compares the upstream and downstream oxygen sensor signals on Bank 1 and determines that the warm-up catalyst is not storing oxygen or reducing emissions as expected. During the first few minutes of operation, the catalytic converter must heat up quickly to meet emissions standards. When efficiency drops below a calibrated threshold, the ECU logs P0421.
This code is common on high-mileage vehicles, engines with oil consumption problems, and vehicles that experience frequent short trips where the catalyst never fully heats up. It often follows earlier issues such as misfires, fuel trim problems, or exhaust leaks.
Quick Reference
- OBD-II Family: P-Code (Powertrain)
- Scope: Generic
- System: Emissions Control / Catalyst Efficiency
- Difficulty Level: Intermediate to Advanced
- Estimated Repair Cost: €120–€900
- Last Updated: 2025-11-24
Real-World Example / Field Notes
On certain Mitsubishi, Hyundai, Toyota, and GM vehicles, P0421 commonly shows up after a period of misfires or a long-term rich-running condition. One Mitsubishi Outlander 2.4L case involved a driver who ignored a misfire for several weeks. The unburned fuel overheated and damaged the warm-up catalyst, causing P0421 to appear soon after the misfire was fixed. Replacing the upstream O2 sensor alone didn’t help — the catalytic converter substrate was partially melted. A new converter and an upstream O2 sensor finally resolved the issue.
Symptoms of P0421
- Check Engine Light: Usually the only early warning.
- Poor fuel economy: Catalyst inefficiency can increase fuel use.
- Rough cold start: Warm-up emissions strategy may be affected.
- Sulfur or “rotten egg” smell: Catalyst may be saturated or damaged.
- Loss of power: In severe cases, a clogged cat reduces airflow.
- Rattling noise: Broken catalyst substrate inside the housing.
Common Causes of P0421
Most Common Causes
- Worn or failing warm-up catalytic converter (Bank 1).
- Failing upstream oxygen sensor sending incorrect data.
- Engine running rich due to fuel trim issues.
- Misfires allowing unburned fuel to enter the catalytic converter.
- Exhaust leaks near the upstream or downstream O2 sensors.
Less Common Causes
- Oil consumption contaminating the catalyst.
- Coolant leaking through a failing head gasket and damaging the catalytic converter.
- Downstream oxygen sensor aging or slow response.
- ECU calibration errors (rare; sometimes resolved with software updates).
- Aftermarket catalytic converters with insufficient efficiency.
Diagnosis: Step-by-Step Guide
Begin by confirming that the catalytic converter’s performance is truly below threshold and not being misread due to sensor issues.
Tools You’ll Need: OBD-II scanner with live data, infrared thermometer, vacuum gauge, smoke tester (optional), multimeter, and basic hand tools.
- Scan for related codes. Look for misfires (P0300–P030X), fuel trim issues, or O2 sensor codes. Fix these first.
- Check fuel trims. Rich or lean conditions accelerate catalytic converter wear.
- Inspect for exhaust leaks. Leaks before the upstream or downstream O2 sensors can skew readings.
- Monitor O2 sensor waveforms. Upstream should fluctuate rapidly; downstream should remain relatively steady. If they match closely, the catalyst may be weak.
- Check catalyst temperature. Using an infrared thermometer, Bank 1 outlet temperature should be higher than the inlet at normal operating temperature.
- Verify O2 sensor response time. Lazy sensors can mislead the ECU into thinking the cat is bad.
- Inspect for oil or coolant burning. Blue or white smoke indicates internal engine damage contributing to catalyst failure.
- Check backpressure. A severely restricted catalytic converter will cause high exhaust backpressure.
- Use Mode $06 data if available. It can reveal borderline catalyst performance ratios before they become obvious in live data.
- Confirm catalyst condition. If all sensors and fuel trims are correct, the converter itself is likely worn or damaged.
Pro Tip: Compare Bank 1 and Bank 2 downstream oxygen sensor activity on V6/V8 engines. If Bank 1’s downstream sensor tracks the upstream signal too closely, the catalytic converter on that bank is likely failing.
Possible Fixes & Repair Costs
- Replace warm-up catalytic converter (Bank 1): The most common fix (€250–€900 depending on vehicle).
- Replace upstream O2 sensor: If it is slow or inaccurate (€60–€180).
- Fix misfires: Prevents further catalyst damage (€20–€250 depending on cause).
- Repair exhaust leaks: Corrects false readings (€50–€150).
- Repair fuel trim issues: Resolves long-term rich running (€40–€200 for sensors or cleaning).
- Replace downstream O2 sensor: Only if proven faulty (€60–€150).
Always verify the cause before replacing the catalytic converter — a misdiagnosed sensor can lead to unnecessary repairs.
Can I Still Drive With P0421?
You can often drive with P0421 for a short time without immediate damage, but the vehicle may consume more fuel and fail emissions testing. If you notice loss of power, rattling from the exhaust, or heavy fuel smell, stop driving to prevent catalytic converter breakup, which can clog the exhaust and make the car undriveable.
Related Codes
- P0474 – Exhaust Pressure Sensor Intermittent
- P0473 – Exhaust Pressure Sensor High
- P0472 – Exhaust Pressure Sensor Low
- P0471 – Exhaust Pressure Sensor Range/Performance
- P0470 – Exhaust Pressure Sensor
- P0469 – EVAP Purge Flow Sensor Circuit Intermittent
- P0468 – EVAP Purge Flow Sensor Circuit High
- P0467 – EVAP Purge Flow Sensor Circuit Low
- P0466 – EVAP Purge Flow Sensor Circuit Range/Performance
- P0465 – EVAP Purge Flow Sensor Circuit
Key Takeaways
- P0421 indicates the warm-up catalytic converter on Bank 1 is not working efficiently.
- Common causes include catalyst wear, O2 sensor faults, rich running, or misfires.
- Driving is possible short-term, but long-term damage or clogging is a risk.
- Always diagnose fuel trims and O2 sensor behavior before replacing the converter.
FAQ
What causes P0421 to appear?
The most common causes are a failing catalytic converter, exhaust leaks, misfires, or a weak upstream O2 sensor. Long-term rich running can rapidly degrade the catalyst.
Can an O2 sensor cause P0421?
Yes. A slow or inaccurate upstream O2 sensor can mimic catalyst failure by feeding incorrect data to the ECU. Always check sensor response before replacing the converter.
Is P0421 the same as P0420?
Similar, but not identical. P0421 specifically refers to the warm-up catalyst efficiency during initial engine operation, while P0420 measures overall catalyst efficiency.
Can I keep driving with P0421?
You can for a short period, but extended driving may lead to increased fuel consumption, emissions issues, or catalytic converter clogging. Address the problem as soon as possible.
Will replacing the catalytic converter fix P0421?
Often yes, but only if the root cause is addressed. If a misfire or rich condition remains, the new converter will fail again prematurely. Diagnose thoroughly before replacing parts.