P0420 Subaru Forester: Catalyst System Efficiency Below Threshold (Bank 1)

Subaru Forester P0420 Catalytic Converter Diagnosis

Quick Answer: What is P0420?

On a Subaru Forester, the P0420 code indicates that the catalytic converter is not processing exhaust gases efficiently. Due to the Boxer engine design, Subaru exhaust systems are prone to small leaks at the manifold gaskets or "donut" gaskets, which often trick the sensors into setting this code even if the converter is still functional.

Pro Tip: Subaru's Engine Management System is extremely sensitive. Often, a P0420 is triggered by a "lazy" Rear Oxygen Sensor (Sensor 2) before the converter has actually failed. Replacing the sensor first can save you over $800.

Common Symptoms

Check Engine Light: On with code P0420.
Cruise Control Disabled: On newer Foresters, the Cruise Control light will flash and the system will be disabled when an emissions code is active.
Failed Emissions: Cannot pass state inspections (Smog check).
Slight Sulfur Smell: Noticeable odor after high-speed driving.

Most Likely Causes

Exhaust Gasket Leak (35%): Small leaks at the exhaust manifold gaskets allow air to enter, skewing oxygen readings.
Failed Catalytic Converter (35%): Internal structure degradation, common after 150,000 miles.
Faulty Rear O2 Sensor: The sensor is sending an inaccurate signal to the computer.
Engine Oil Consumption: If your Forester has a head gasket or piston ring leak, burned oil can "poison" the catalyst.

Detailed Repair Cost Breakdown

Estimates for Subaru Forester (2.5L and 2.0L engines) in the US market:

Component / Task	Aftermarket Part	OEM Subaru Part	Labor Time
Catalytic Converter Assembly	$350 - $600	$900 - $1,400	1.5 - 2.5 hrs
Rear O2 Sensor (Denso)	$65 - $100	$180 - $240	0.5 hr

How to Fix P0420 on a Subaru Forester

1. Check the Manifold Gaskets

With the engine cold, spray soapy water on the exhaust manifold gaskets and flange joints. Start the engine briefly. If you see bubbles, you have an exhaust leak. Replacing a $20 gasket is the most common "cheap fix" for a Subaru P0420 code.

2. Only Use OEM or Denso Sensors

Subaru electronics are notoriously picky. If you decide to replace an oxygen sensor, do not use a universal brand. Use only Genuine Subaru or Denso (the original manufacturer) sensors to ensure the code does not return immediately.

3. Try a Catalytic Converter Cleaner

Since the Boxer engine can run slightly rich, carbon buildup on the catalyst is common. A high-quality fuel system and cat cleaner additive can sometimes clear enough residue to turn the light off and allow you to pass a state inspection.