I don't memorize code numbers but 50% is easy it is for a dead O2 sensor. It's easy because all cars exhibit 50% until the O2 sensor warms up. Sometimes at idle they even cool enough to go back to 50%. A bad heater circuit will contribute greatly to slow sensors

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician