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BMW Fault Codes P1421 P1423 – Secondary Air

August 26, 2010

This answer is applicable for many BMWs in addition to the one listed below.

Q
car year: 1997
car model: 328is
My BMW has failed emission test and I have 2 codes. Can you tell me what those codes mean and the part to fix it? P1423 and P1421.

A
These two codes are for the Secondary Air System, bank-1 and bank-2. Bank one is cylinders 1 through 3 and bank-2 is cylinders 4 through 6. These codes are indicating that the secondary air system is not functioning properly.

The secondary air system pumps fresh air into the exhaust ports during cold engine start-up. This provides fresh oxygen to assist in the complete burning of the combustion mixture during the cold start-up period, when the mixture is richer and is not burning efficiently.

Commonly, the check valve at the exhaust manifold will fail (stick open) and this will allow exhaust gasses to enter the pump when it is not running. This will destroy the pump. The typical repair involves replacing the pump and the check valve (and the gasket for the check valve). Of course, there are other areas within the secondary air system that could be at fault. For further information and detailed diagnostic steps, please see the Summer 2008 issue of our Fast Times newsletter (below).

Summer 2008 Fast Times newsletter:

Secondary Air System components:

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