What causes P0420 on a Subaru Forester?

Common causes of P0420 on Subaru Forester: Catalytic converter failure is a well-known issue on Subaru boxer engines (all models); Downstream O2 sensor contamination from head gasket oil/coolant weep on EJ25 engines; Exhaust header/manifold gasket leak from boxer engine thermal cycling; Catalyst substrate cracking from uneven exhaust pulses inherent to flat-four design; Oil consumption past valve seals contaminating catalyst on FB25 engines.

P0420

P0420 Subaru Forester

Q: How much does it cost to fix P0420 on a Subaru Forester?

Fixes for P0420 on Subaru Forester include: Replace catalytic converter with Subaru OE or CARB-compliant unit designed for boxer engines, Replace downstream O2 sensor with Denso OE and fix head gasket weep if present on EJ25, Replace exhaust header gaskets on both sides of boxer engine, Replace both bank catalytic converters simultaneously if both are marginal on inspection, Address valve seal oil consumption before replacing catalytic converter on FB25.

Catalyst System Efficiency Below Threshold (Bank 1) · Powertrain Code · Medium

What is P0420?

The catalytic converter on bank 1 is not operating efficiently. The downstream O2 sensor readings indicate the catalyst is not converting exhaust gases properly. May need catalyst replacement or could be caused by a failed O2 sensor.

When the Subaru Forester triggers code P0420 (catalyst system efficiency below threshold (bank 1)), it typically points to an issue in the powertrain system. Below you'll find the most common causes, symptoms, and repair options specific to the Subaru Forester.

Diagnostic data on this page is compiled from publicly available repair resources and technical service bulletins. Always have a qualified mechanic confirm the diagnosis before authorizing repairs on your Subaru Forester.

Common Causes on Subaru Forester

Catalytic converter failure is a well-known issue on Subaru boxer engines (all models)
Downstream O2 sensor contamination from head gasket oil/coolant weep on EJ25 engines
Exhaust header/manifold gasket leak from boxer engine thermal cycling
Catalyst substrate cracking from uneven exhaust pulses inherent to flat-four design
Oil consumption past valve seals contaminating catalyst on FB25 engines

Symptoms

Check engine light illuminated steadily
Reduced fuel economy compared to normal
Sulfur or rotten egg smell from exhaust
Failed emissions inspection or smog test
Slight loss of power at highway speeds
Rough or uneven exhaust note

Recommended Fixes

Replace catalytic converter with Subaru OE or CARB-compliant unit designed for boxer engines

Replace downstream O2 sensor with Denso OE and fix head gasket weep if present on EJ25

Replace exhaust header gaskets on both sides of boxer engine

Replace both bank catalytic converters simultaneously if both are marginal on inspection

Address valve seal oil consumption before replacing catalytic converter on FB25

Notes for Subaru Forester

Subaru boxer engines have higher-than-average cat failure rates; Subaru extended warranty coverage may apply.

P0420 Subaru Forester

What is P0420?

Common Causes on Subaru Forester

Symptoms

Notes for Subaru Forester

Check Subaru Forester Specs