P0420 on 2019-2021 Subaru Ascent: Catalyst Efficiency Causes and Fixes

What's Unique About the 2019-2021 Subaru ASCENT

The 2019-2021 Subaru Ascent uses the 2.4L FA24F turbocharged direct-injection engine. While P0420 is a common code across all Subaru models, the specific diagnostic procedure for the Ascent is outlined in Technical Service Bulletin (TSB) #09-110-23R. This TSB indicates Subaru has identified a need for a specific, detailed diagnostic path for technicians to follow. The procedure explicitly requires checking for other DTCs, verifying engine oil levels, checking for white smoke (oil/coolant consumption), and inspecting for exhaust leaks *before* condemning the catalytic converter. This suggests that misdiagnosis is common and that other factors, including engine mechanical issues, are often at play.

Diagnostic Flowchart

Tap your situation to follow the diagnostic path that matches what you're seeing on this vehicle.

Symptoms You May Notice

• Check Engine Light is on
• Blinking cruise control light (a common Subaru feature when a CEL is active)
• Disabled traction control light
• Failure to pass an emissions test
• Reduced engine power or sluggish acceleration if the converter is clogged
• Noticeable drop in fuel economy

Most Likely Causes

1. Failing Catalytic Converter 🔴 High Probability Catalytic converters can be contaminated by other engine issues like burning oil or rich fuel mixtures, which can damage the internal catalyst materials. While not necessarily a design flaw, it's the most direct cause of the code. The Federal Emissions Warranty covers this part for 8 years or 80,000 miles.
   How to confirm: A technician can use a scan tool to monitor O2 sensor data; a rear sensor that mimics the front sensor suggests a bad converter. An infrared thermometer can also be used to check the converter's inlet and outlet temperatures; the outlet should be significantly hotter on a working converter. TSB 09-110-23R provides a specific road test procedure for dealers to confirm failure.
   Typical fix: Replace the catalytic converter assembly. It is critical to address any underlying engine issues (like oil consumption) first, or the new converter will also fail. Many owners have had this replaced for free under the federal emissions warranty.
   Est. part cost: $1500-$2500
2. Faulty Oxygen Sensor (Rear/Downstream) 🟡 Medium Probability Oxygen sensors can become 'lazy' or fail with age, sending incorrect data to the ECM. This can trick the computer into thinking the converter is bad when it's actually the sensor that's faulty. It is a much cheaper part to replace and a common first step in diagnosis after checking for leaks.
   How to confirm: Graph the sensor's voltage output with a scan tool. A healthy downstream sensor should show a relatively steady voltage on a warm engine. A faulty one might fluctuate wildly, be stuck at a certain voltage, or not respond at all. The sensor harness should also be checked for damage.
   Typical fix: Replace the rear (downstream) oxygen sensor.
   Est. part cost: $80-$200
3. Exhaust System Leak 🟡 Medium Probability Leaks in the exhaust manifold, gaskets, or flex pipe ahead of or near the catalytic converter can allow outside oxygen to enter the exhaust stream. This extra oxygen skews the O2 sensor readings, leading to a false P0420 code. TSB 09-110-23R specifically lists checking for leaks as a key diagnostic step.
   How to confirm: Perform a visual inspection for cracks or soot trails around exhaust components. A smoke test, where smoke is forced into the exhaust system, is the most effective way to pinpoint small leaks.
   Typical fix: Repair the leak by replacing the faulty gasket or welding the cracked component.
   Est. part cost: $20-$300
4. Faulty Air-Fuel Ratio Sensor (Front/Upstream) ⚪ Low Probability A slow or failing front A/F sensor can cause the engine to run slightly rich or lean. This not only affects the converter's efficiency but can also lead to its premature failure over time by sending unburned fuel into the exhaust.
   How to confirm: Graph the sensor's readings on a scan tool. It should oscillate rapidly. A slow or stuck reading indicates a problem. TSB 09-110-23R includes a step to check A/F sensor values.
   Typical fix: Replace the front (upstream) air-fuel ratio sensor.
   Est. part cost: $100-$250

Rare But Worth Checking

• Engine Misfire or Rich/Lean Condition: If the engine is misfiring or has a fuel mixture problem, it can send unburned fuel into the exhaust, which can overheat and damage the catalytic converter. This would typically be accompanied by other codes (e.g., P030x for misfires, P0171/P0172 for fuel trim). TSB 09-110-23R requires diagnosing these codes first.
• ECM Software Issue: On some Subaru models, older ECM software could be too sensitive and trigger a P0420 code prematurely. While no specific software update TSB for the Ascent P0420 is noted, older bulletins for other models have involved ECM reprogramming to fix this issue, making it a possibility.
• Excessive Oil Consumption: Some Subaru engines have been prone to oil consumption issues, which can contaminate and destroy a catalytic converter over time. TSB 09-110-23R specifically instructs technicians to check for white smoke from the tailpipe, an indicator of burning oil or coolant.

Diagnosis Steps

1. Verify the code with an OBD-II scanner.
2. Check for any other stored trouble codes that could indicate an underlying engine problem (e.g., misfires, fuel trim issues). Per TSB 09-110-23R, these must be addressed first.
3. Check the engine oil level and look for signs of white or blue smoke from the tailpipe, which indicates oil or coolant consumption that can damage the converter.
4. Inspect the entire exhaust system for leaks from the engine to the rear O2 sensor. Look for black soot, listen for hissing sounds, or perform a smoke test.
5. Use a scan tool to graph the live data from both the upstream (Bank 1, Sensor 1) and downstream (Bank 1, Sensor 2) oxygen sensors.
6. Confirm the upstream sensor is oscillating rapidly. The downstream sensor should have a much steadier, flatter voltage reading on a warm, running engine at steady RPM.
7. If the downstream sensor's reading mirrors the upstream sensor, the catalytic converter is likely inefficient.
8. If the downstream sensor's reading is flat, stuck, or erratic, the sensor itself may be faulty. Wiggle the sensor harness to check for intermittent connections.
9. If no other cause is found, test the catalytic converter directly using an infrared thermometer to compare inlet and outlet temperatures. The outlet should be at least 100°F hotter than the inlet on a fully warmed-up engine.
10. Contact a Subaru dealer to check for warranty coverage. The catalytic converter is covered under the Federal Emissions Warranty for 8 years or 80,000 miles.

Parts You'll Likely Need

• Catalytic Converter (OEM #44621AA33A) — This is the most common, though most expensive, fix if the converter has truly failed and lost its efficiency. Covered by federal emissions warranty for 8 years/80,000 miles.
  Trusted brands: Subaru (OEM)
  OEM price range: $2000-$2500
  Aftermarket price range: $800-$1500
• Rear Oxygen Sensor (OEM #22690AB09A) — A faulty or 'lazy' rear O2 sensor can send incorrect data, falsely triggering the P0420 code. It's a common misdiagnosis for a bad converter.
  Trusted brands: Denso (OEM supplier), NGK/NTK, Bosch
  OEM price range: $150-$200
  Aftermarket price range: $80-$130
• Front Air-Fuel Ratio Sensor (OEM #22641AA79A) — A failing front sensor can affect the engine's fuel mixture, which can in turn affect the catalytic converter's performance and trigger the code.
  Trusted brands: Denso (OEM supplier), NGK/NTK
  OEM price range: $200-$250
  Aftermarket price range: $100-$180

Technical Service Bulletins (TSBs) & Recalls

• 09-110-23R: Announces diagnostic service procedures to be followed when DTC P0420 (Catalyst System Efficiency Below Threshold Bank 1) is detected by the Engine Control Module (ECM). It details a multi-step diagnostic flow chart that prioritizes checking for other codes, oil consumption, and exhaust leaks before testing sensors or replacing the converter.

Platform-Specific Known Issues

• Subaru issued TSB #09-110-23R specifically to guide technicians through a detailed diagnostic procedure for P0420 on the 2019-2021 Ascent, among other models. This bulletin emphasizes a step-by-step process, including checking for other DTCs, inspecting for white smoke (oil consumption), and testing sensor harnesses, before condemning the catalytic converter.
• The Federal Emissions Defect Warranty provides coverage for the catalytic converter for 8 years or 80,000 miles, whichever comes first. Many owners who get the P0420 code within this period may be eligible for a free replacement at a Subaru dealership.