P0420 on Subaru vehicles
Catalyst Efficiency Below Threshold — what it means on a Subaru, whether it's safe to drive, and the fixes by model and year.
P0420 on a Subaru means catalyst efficiency below threshold. Most vehicles stay drivable short-term, but diagnose it promptly. The most common cause is worn or contaminated catalytic converter (bank 1) (typically $600–$2,400). Pick your Subaru model below for its known failure patterns, recalls, and TSBs.
What does P0420 mean on a Subaru?
P0420 is set when the engine control module (ECM) compares the upstream and downstream oxygen sensor readings on Bank 1 and concludes the catalytic converter is no longer storing and releasing oxygen efficiently. In a healthy system, the downstream O2 sensor signal stays relatively flat compared to the switching upstream sensor. When both signals look similar, the ECM infers the catalyst has lost its ability to convert hydrocarbons, carbon monoxide, and oxides of nitrogen into less harmful byproducts.
P0420 is a standardized OBD-II code, so its definition is the same on every Subaru. What differs by model and engine is which cause is most likely — pick your Subaru model below for a diagnosis tuned to its known failure patterns, recalls, and TSBs.
Is it safe to drive a Subaru with P0420?
In most cases a Subaru stays drivable for short trips with P0420 active, but diagnose and repair it promptly. This is a moderate-severity code — ignoring it can lead to further damage or a failed emissions test. Exact tolerance depends on the model and engine.
What causes P0420 on a Subaru?
| Cause | Likelihood | Estimated repair (USD) |
|---|---|---|
| Worn or contaminated catalytic converter (Bank 1) — Most common root cause once the vehicle is over 100,000 miles. | Most common | $600–$2,400 |
| Failing downstream (post-catalyst) oxygen sensor — Slow-switching O2 sensor mimics a dead catalyst. | Common | $150–$450 |
| Exhaust leak upstream of the downstream O2 sensor | Common | $100–$500 |
| Engine misfire or rich/lean fuel trim contaminating the catalyst — Resolve any P0300-series codes before condemning the catalyst. | Occasional | $200–$1,200 |
| Coolant or oil entering the exhaust (head gasket, valve seals) — Phosphorus and silicon poison the catalyst substrate. | Rare | $1,200–$3,500 |
| Aftermarket / off-brand catalytic converter that does not meet OEM spec | Occasional | $600–$1,800 |
Repair costs are typical US ranges and vary by Subaru model, model year, and labor rate. A diagnostic trouble code is a symptom, not a guaranteed failed part — confirm the root cause before replacing anything.
P0420 by Subaru model
Select your Subaru model and year for a P0420 diagnosis tuned to that vehicle's failure patterns, recalls, and TSBs.
Subaru Outback
1,651 NHTSA owner complaints · 24 recalls on record across the years covered below.
Subaru Forester
765 NHTSA owner complaints · 17 recalls on record across the years covered below.
Complaint and recall counts are NHTSA totals for the model years covered on this site, not counts specific to P0420.
P0420 on other makes
Other codes on Subaru vehicles
P0420 on a Subaru: frequently asked questions
What does diagnostic trouble code P0420 mean on a Subaru?
P0420 is set when the engine control module (ECM) compares the upstream and downstream oxygen sensor readings on Bank 1 and concludes the catalytic converter is no longer storing and releasing oxygen efficiently. In a healthy system, the downstream O2 sensor signal stays relatively flat compared to the switching upstream sensor. When both signals look similar, the ECM infers the catalyst has lost its ability to convert hydrocarbons, carbon monoxide, and oxides of nitrogen into less harmful byproducts. P0420 is a standardized OBD-II code, so the definition is the same on every Subaru — which cause is most likely varies by model and engine.
What are the symptoms of P0420 on a Subaru?
Check Engine Light is illuminated (steady, not flashing). Mild loss of fuel economy. Faint rotten-egg (sulfur) smell from the exhaust. Slight reduction in power under hard acceleration. Vehicle will fail an OBD-II emissions / smog test. Typically no noticeable drivability issue at idle
What causes P0420 on a Subaru?
Worn or contaminated catalytic converter (Bank 1) (most-common). Failing downstream (post-catalyst) oxygen sensor (common). Exhaust leak upstream of the downstream O2 sensor (common). Engine misfire or rich/lean fuel trim contaminating the catalyst (occasional). Coolant or oil entering the exhaust (head gasket, valve seals) (rare). Aftermarket / off-brand catalytic converter that does not meet OEM spec (occasional)
Is it safe to drive a Subaru with P0420?
In most cases a Subaru stays drivable for short trips with P0420 active, but it should be diagnosed and repaired promptly — this is a moderate-severity code. Ignoring it can lead to further damage or a failed emissions test. Exact tolerance depends on the model and engine.