Diagnostic trouble code P0306 on your 2020 Subaru Legacy indicates a cylinder 6 misfire, but this code requires special diagnostic attention. The 2020 Legacy marked the first model year without a six-cylinder engine option, as Subaru discontinued the 3.6R flat-six after 2019. All 2020 Legacy models use four-cylinder engines.
2020 Legacy Engine Options
The 2020 Subaru Legacy introduced a new platform with two four-cylinder engine choices: a 2.5-liter naturally aspirated boxer producing 182 horsepower, and a 2.4-liter turbocharged boxer delivering 260 horsepower. Neither engine has a sixth cylinder, making P0306 an impossible legitimate code for this model year.
Why P0306 Appears Incorrectly
Since the 2020 Legacy physically lacks a cylinder 6, this code represents a diagnostic system malfunction. Possible causes include ECM software bugs or programming errors, scan tool communication issues, aftermarket electronic device interference, or ECM hardware faults. The system is reporting a condition that cannot physically exist.
Diagnostic Investigation
Verify the code with a different scan tool, preferably Subaru's SSM system. Remove any aftermarket OBD-II devices. Check ECM connectors and grounds for issues. If your Legacy exhibits actual misfire symptoms like rough running or hesitation, focus diagnosis on cylinders 1-4 as the real source of any problems.
Historical Context
If you're accustomed to older Legacy models with the 3.6R engine, P0306 would have been a valid code on 2019 and earlier models. The 2020 redesign eliminated this engine option, so P0306 is now anomalous for Legacy models of this generation.