P0420 on a 2014-2015 Subaru Forester 2.5L: Check the Exhaust for Leaks First

P0420 on a 2014-2015 Subaru Forester 2.5L (FB25) means the ECM judges that the front catalytic converter is not cleaning the exhaust well enough. Start with an exhaust leak near the converter or sensors, since that is the cheapest thing to rule out. Confirm by graphing the rear oxygen sensor against the front A/F sensor on a steady highway drive. If the engine runs rough or stalls, stop driving it and diagnose.

What does P0420 mean on a 2014-2015 Forester?

P0420 (Catalyst System Efficiency Below Threshold, Bank 1) points at the front catalytic converter, which is built into the front exhaust pipe. The ECM judges it using the front A/F sensor upstream and the rear oxygen sensor downstream.

The ECM adds up how much the front A/F lambda value and the rear oxygen sensor voltage move, in 32 ms steps, four times per calculation, then divides the rear total by the front total. A healthy converter keeps the rear signal calm while the front moves. A worn one lets the rear follow the front, so the ratio climbs.

The ECM sets P0420 when the rear deviation sum divided by the front deviation sum is above 10 (above 8 on PZEV models), over a diagnosis time of 30 to 55 seconds. The check only runs when all of these hold:

• Battery voltage 10.9 V or more.
• Atmospheric pressure 75 kPa (22.2 inHg) or more.
• Coolant temperature 60 degrees C (140 degrees F) or more.
• Estimated catalyst temperature 535 degrees C (995 degrees F) or more, CVT and manual alike.
• Misfire count per 200 rotations below 65535.
• EVAP gas density learning value below 1.
• Sub feedback in operation.
• Evaporative system diagnosis not running.
• Time with actual lambda within 0.10 of target lambda 0 ms or more.
• Vehicle speed above 40 km/h (24.9 mph).
• Intake air amount 8 g/s (0.28 oz/s) or more and below 45 g/s (1.59 oz/s).
• Engine load change per 0.5 engine revolutions below 255 g/rev (8.99 oz/rev).
• Rear oxygen sensor output has crossed from 660 mV or less to 660 mV or more after a fuel cut.
• Elapsed time after start and purge execution time both 0 s or more.

The diagnosis runs once, at a constant 40 km/h (24.9 mph) or higher. The fault clears when the same ratio comes back at 10 or lower (8 or lower on PZEV) in a later check.

The fault is detected when two consecutive driving cycles show it, and the warning light comes on when the malfunction occurs in 2 continuous driving cycles.

Two groups of related codes matter here:

• Rear oxygen sensor codes: P0137 (low voltage, bank 1 sensor 2) sets with output below 0.03 V, and P0138 (high voltage) sets above 1.2 V. P0140, P013A, P013B, P013E and P013F cover the same sensor.
• Post catalyst fuel trim codes: P2096 (too lean, bank 1) sets with a sub feedback learning value below minus 0.042 on CVT or minus 0.044 on manual, non-PZEV. P2097 (too rich) sets at 0.013 or more on CVT or 0.017 or more on manual, non-PZEV.

What symptoms does P0420 cause on the Forester?

• Check engine light on.
• Engine stalls.
• Idle mixture out of specification.
• You may notice reduced fuel economy.
• You may notice a sulfur or rotten egg smell.
• You may hear a rattle from the exhaust.
• A failed emissions test.

What causes P0420 on the Forester, most likely first?

Here is how I would rank the causes.

How do you diagnose P0420 on a 2014-2015 Forester?

You will need a scan tool that graphs live data, basic hand tools, a torque wrench, spray lubricant, anti-seize and new exhaust gaskets.

Snug the head nuts first, fit the center to rear pipe bolts and the hanger bolt, then tighten the head nuts to torque. Spray lubricant on a sensor thread and wait a minute or more before removing it. Anti-seize goes on sensor threads only, never on the protector.

What if the obvious fix does not work?

Ask whether the converter was ever the real problem. Replacing it before the cheaper checks is a common mistake, and so is replacing only the sensors when the converter has failed. Rule out leaks near the sensors first, including intake or vacuum leaks, though the code can return after a leak repair if the converter is the cause.

An aging or contaminated rear oxygen sensor can make a healthy converter look bad. If you replace it, an OEM or OEM supplier sensor is the safer choice, since cheaper ones are a possible source of repeat codes. Spacers or other tricks that change what the sensors read hide the code without repairing anything.

If the oil level keeps dropping or you see white smoke at the muffler, stop the converter work. The engine repair comes first, then the front converter.

How do you confirm the repair?

Clearing memory with the scan tool after the repair is required. Wait 10 seconds or more with the ignition ON before starting the engine, and clear memory again after reconnecting any sensor connector. Check any other stored code before you replace the converter, and record the codes before repairing.

Inspection mode does not test P0420, so use the drive cycle instead:

• Battery at 12 V or more, and fuel about half full (20 to 40 L, 5.3 to 10.6 US gal).
• Clear memory and confirm no code remains.
• Warm the engine.
• Drive 20 minutes or more at a constant 80 km/h (50 mph) or more.
• Stop and idle for one minute.
• Read the temporary code with the scan tool.

Then repeat the highway graph and confirm the rear trace is back to normal.