Subaru

Subaru OBD Codes
boxer engine faults explained

The codes Subaru owners actually see — AVCS cam timing, misfires, catalysts and O2 sensors — with clear causes and fixes.

Subaru P-codes Causes & fixes Free decoder
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Part 1 · The basics

Subaru check engine light codes

Subaru uses standard OBD-II engine codes. Its boxer engines use AVCS (active valve control system), which produces distinctive cam-timing codes like P0011 and P0021.

Here are the codes most reported on Outback, Forester, Impreza, WRX and Crosstrek models.

Part 2 · Decode

OBD-II code decoder

System
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Type
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Subsystem
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Fault #
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Part 3 · The dictionary

Most common Subaru

Classic Subaru faults behind these codes.

Code Description
P0011 "A" Camshaft Position Timing Over-Advanced (Bank 1)
P0021 "A" Camshaft Position Timing Over-Advanced (Bank 2)
P0133 O2 Sensor Slow Response (Bank 1, Sensor 1)
P0171 System Too Lean (Bank 1)
P0300 Random/Multiple Cylinder Misfire
P0420 Catalyst System Efficiency Below Threshold (Bank 1)
P0442 EVAP System Small Leak Detected
P0507 Idle Control System RPM Higher Than Expected
Part 4 · Common faults

Common Subaru faults

Classic Subaru faults behind these codes.

1

AVCS timing (P0011/P0021) — oil pressure or oil control valve issues on boxer engines.

2

Misfires (P0300) — coils, plugs, or head-gasket-adjacent coolant issues on older engines.

3

Catalyst efficiency (P0420) — aging converter, common on higher-mileage models.

4

O2 sensor slow (P0133) — upstream sensor wear.

5

EVAP leaks (P0442) — fuel cap or hoses.

Part 4 · Common faults

Subaru FAQ

What causes P0011/P0021 on a Subaru?
The AVCS cam timing is over-advanced — usually low oil, a clogged oil control valve, or an oil pressure issue. Check oil level and change the oil before replacing parts.
Why does my Subaru misfire?
Ignition coils and spark plugs are the most common cause. On older boxer engines, monitor coolant and temperature — head gasket issues can also cause misfires.
Is P0420 common on Subaru?
Yes, at higher mileage. Confirm with O2 sensor data and an exhaust leak check before replacing the converter.
Can I read Subaru codes with a generic scanner?
Yes, engine codes are standard OBD-II. AVCS and module-level data benefit from Subaru Select Monitor or a good professional scanner.
Are the codes different on WRX vs Outback?
No — Subaru shares OBD-II engine codes across the range; the same P-code means the same fault.

Codes vary slightly by model, year and engine. Always confirm with a proper diagnosis. Manufacturer-specific codes (P1xxx/P2xxx) may differ from generic descriptions.

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