A P0420 code in the 2022 Ford Expedition indicates the engine computer has detected that the Bank 1 catalytic converter isn't cleaning exhaust as efficiently as expected. Before assuming the converter needs replacement, proper diagnosis can identify whether the catalyst is truly failing or another issue is causing the code.
How the Computer Monitors Catalyst Efficiency
The Expedition uses oxygen sensors before and after the catalytic converter on each exhaust bank. The upstream sensor sees the raw exhaust variations as cylinders fire. The downstream sensor should see a relatively steady signal because the converter stores and releases oxygen, smoothing out the variations.
When the downstream sensor's signal starts mirroring the upstream sensor's fluctuations, it indicates the converter's oxygen storage capacity has degraded. The computer sets P0420 when efficiency drops below acceptable thresholds.
True Converter Failure vs. Other Causes
Catalytic converters can genuinely fail from age, but in a 2022 vehicle, premature failure suggests underlying issues. Misfires send unburned fuel to the converter, causing extreme heat damage. Oil consumption coats the catalyst with deposits. Running rich floods the converter with fuel it can't process.
Before replacing an expensive converter, verify no misfire codes are present, no oil consumption issues exist, and the engine isn't running rich. Addressing these root causes prevents damaging a new converter.
Oxygen Sensor Considerations
A lazy downstream oxygen sensor can cause P0420 by incorrectly reporting converter operation. If the sensor is slow to respond to changes, it may trigger efficiency codes even with a good converter. Sensor testing with a scan tool monitoring response rates can identify lazy sensors.
Converter Replacement on the Expedition
The 3.5L EcoBoost V6 has two catalytic converters, one per exhaust bank. Bank 1 (P0420) is typically the passenger side. OEM converters are expensive ($1,500-2,500+), and the 2022 model year should still be under the federal emissions warranty (8 years/80,000 miles for emissions components).
If outside warranty, quality aftermarket converters are available but must meet EPA requirements and may not trigger coverage for future issues. Cheap converters often fail quickly.