How to Fix P0420 Code on 2004 Subaru Outback with California Emissions Package?

The catalytic converter is likely faulty. When the downstream O2 sensor shows oscillation patterns similar to the upstream sensor—especially in cross-counts—it indicates that the converter has degraded and needs replacement.

What should the output of the rear O2 sensor look like? I'm not familiar with interpreting O2 sensor waveforms. Where can I find reliable information on expected readings or patterns for this setup? I’m trying common, low-cost fixes first since catalytic converters are expensive.

It's the catalytic converter. Avoid 'cheap fixes'—they don’t address the root cause.

I've removed all quick-fix solutions and invested in a better OBD-II reader with graphing capabilities. Now I want to verify whether the rear O2 sensor readings are truly indicating a failing converter or just a sluggish sensor. Before replacing the catalytic converter, I’d like to confirm it's actually the converter versus an aging sensor. I’m not sure if I'm wasting time—but knowing what I'm fixing and seeing measurable changes helps me make informed decisions. What should the expected output look like? Thanks, Jeremy.

In graph mode, observe both upstream and downstream O2 sensors at idle and around 2000 RPMs. The upstream O2 sensor should show a sharp, choppy sine wave due to fuel-air mixture fluctuations. In contrast, the downstream O2 sensor should display a nearly flat or slow-moving waveform—indicating effective catalytic conversion. If the downstream signal closely mirrors the upstream pattern, it strongly suggests a failing catalytic converter.

I’ve been dealing with P0420 on my own vehicle too. A local Subaru dealer suggested clearing the code and monitoring how long it returns—during that time, run fuel cleaner through the system to clean injectors and oxygen sensors. You mentioned applying anti-foulers to the rear O2 sensor? If it's located near the rear of the exhaust, consider instead placing them on the front O2 sensor (closer to the engine). However, this is incorrect—front O2 sensors regulate fuel trim; modifying them does not affect catalytic converter performance and could cause unintended issues. This advice is misleading and potentially illegal.

Putting anti-foulers on front O2 sensors is a flawed recommendation. Front O2 sensors are used for fuel control, not catalyst monitoring. Suggesting this would create more problems than solutions and does nothing to fix the P0420 code. If you don’t understand how emissions systems work, please refrain from offering advice—it only causes confusion. This is a dangerous and illegal workaround.

O2 sensors with over 100,000 miles should be replaced—especially preheated models (common in vehicles from the late 1990s onward), which typically last between 60,000 to 80,000 miles. Older non-preheated sensors may fail earlier and can give false lean readings, causing the engine to run richer and overheat the catalytic converter, eventually leading to failure. Anti-foulers are a temporary fix that only work if you're removing the converter—which is illegal. These are band-aid solutions, like using duct tape or body filler. My recommended approach: replace O2 sensors first (especially if over 100k miles). If the P0420 persists, then replace the catalytic converter. The Subaru shop manual specifies when O2 sensors should be replaced. Note that rear O2 sensors only monitor converter performance—front sensors control fuel mixture. This information may help in diagnosing and resolving the issue.