1998 E36 M3
check engine light came on a few weeks ago

Autozone code reader found P1421 & P1423: BMW specific codes

A search of the net finds that these codes indicate low oxygen in both
exhaust manifolds. Nothing serious though - the oxygen is delivered by
the secondary air pump for about 100 seconds after a cold start - now I
know what that thing is that sounds like a vacuum cleaner.

The pump is actually fine and has no signs of corrosion which can be
caused by exhaust gases coming back through a defective non-return
valve on the manifold.

So I browsed around some more, and found that a relay in the main power
distribution box can be bad, giving these codes. Pulled it and
re-installed the same one, went to Autozone and had the codes reset.

Codes came back a few weeks later - pump still hums normally during a
cold start, so I know it's not a serious problem, but does anyone have
an idea what the problem is. Intermittently bad relay is my best guess.

Cheers