P0420 on a Subaru

What is different about P0420 on a Subaru?

Subaru's EJ25 boxer engine carries a well-known head gasket history, and on some generations that history extends into the exhaust system in a way that is documented to contribute to catalyst efficiency codes.

What Subaru owners run into with P0420

EJ25 head gasket leaks reaching the exhaust: Some generations of the EJ25 flat-four engine, used across many Subaru Outback, Legacy, and Forester models, are documented for external head gasket seepage; coolant or oil reaching the exhaust system over time can foul the catalytic converter and contribute to a genuine P0420, separate from a sensor-only cause.

Downstream sensor and exhaust leak as more common first causes: As with most makes, a lazy downstream oxygen sensor or a small exhaust leak ahead of it remains a more common and cheaper-to-diagnose cause of P0420 on Subaru models than an actual failing catalyst, and should be ruled out first.

What to check first on a Subaru

The order below narrows the generic P0420 list down to what is worth checking first on this make. It is a starting order, not a diagnosis: patterns are generation specific and a vehicle outside those generations may have none of them.

Check in this order

• Check for visible signs of external coolant or oil seepage around the head gasket area on EJ25-equipped models
• Inspect the downstream oxygen sensor and connector for wear or damage
• Inspect the exhaust system for leaks ahead of the downstream sensor
• Consider catalyst condition only after ruling out sensor, exhaust leak, and head gasket seepage

The generic picture behind P0420

The computer compares the oxygen sensor signal before the catalytic converter to the one after it. A healthy converter smooths out and delays the switching pattern of the downstream sensor compared to the upstream one, and this code sets when the downstream signal instead starts to mirror the upstream signal too closely, meaning the converter is no longer storing and releasing oxygen the way a working one does. It is a comparison of the two sensor patterns over a full drive cycle, not a single instant reading.

Everything on the main P0420 page still applies here. This page narrows the cause list to the patterns documented on Subaru vehicles, it does not replace the underlying definition.

The generic version of this code

• P0420 on any vehicle