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The ECU electrically pulls a small solenoid valve
open, allowing manifold vacuum to go to a bigger valve
(the combi-valve), which opens a passage from an
electrical air pump to a passage in the head that goes
to the exhaust passages. The ECU also turns the air
pump on and it blows fresh air into the exhaust. This
is in the first 2 minutes or so after a true cold
start. It helps finish burning the rich default
exhaust mixture that is used before the O2 sensor
heats up and the system can go to closed loop
metering. The process of the burn completion also
heats the catalyst faster, resulting in light-off in a
shorter time.
Both valves and the pump are under the intake manifold
behind the radiator. Manifold vacuum comes off a "T"
in the line to the fuel pressure regulator (silver
disc on the fuel rail).
When the car is started and the coolant temperature is
below a certain temperature (80C, I think), the ECU
tests the system by turning it on for a couple of
seconds. This also happens right after the O2 sensor
comes on line after a cold start. Since the O2 is
online, the ECU expects to see at least a 25% leaner
(more oxygen) signal from the O2 (less voltage) while
the pump is running, because it's injecting fresh air
into the exhaust.
Possibilities, in order of likelihood:
1. 50a fuse has corroded and is not longer carrying
current. Check relay/fuse block with relay 111.
Check your air pump gets 12v when the car's cold.
2. Solenoid valve or combi-valve has failed. Air pump
gets 12v and runs but cannot dump air into exhaust
through closed valve. If you can hear your air pump
run this might be the case.
3. Air pump relay has failed.
4. Air pump has failed. From what I hear it makes
nasty bearing noises before it does though.
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