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P0420 - Catalyst System Efficiency Below Threshold (Bank 1). This indicates that the catalyst has failed. The Engine control module (ECM) compares the pre-catalyst and post-catalyst oxygen sensor values to determine it's efficiency. It has failed to meet the parameters.
The catalyst is part of the exhaust manifold on this year model BMW. They retail for about $12,70 (US) and would be about 6 hours labor to replace. If this vehicle has under 80, 000 miles on the odometer and is a US specification vehicle, it should be covered under the federal 8 years / 80,000 mile emissions warranty.
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