Meaning:
The Secondary Air Injection system consists of an air pump which pumps air from the air filter into the exhaust manifold on cold starts to reduce emissions.

On cold startup (and only cold startup), a mixture that is slightly richer than would otherwise be possible is sent to the engine from the ECM. The AIR pump runs, the Combi Valve opens, and additional air is sent into the engine after the exhaust valve to "afterburn" the unburned fuel in the catalitc converter.

This does two things:
1. The burning of this mixture just ahead of the catalytic converter warms it up very fast and brings it to internal operating temperature quickly, which greatly reduces emissions. 2. The slightly richer mixture dramatically improves drivability of a cold engine.

The AIR system typically runs for about a minute or so and then shuts down. A short time later the ECM switches the AIR system back on for a few seconds which sends a blast of air into the exhaust system. The ECM then "looks" for a sudden lean condition at the oxygen sensor to confirm that the AIR system is in fact operating properly (feedback). This does not affect engine running. That's it. Won't come on again until next cold start.

Causes:
Most common problem seems to be hoses. Check the first hose visable in the front top of the engine as soon as you open the hood.
Next, check the combi valve (splits output on V6 to left and right manifold I think)

fix = 1st circuit failure or open fuse
where is this and what does it look like ?

fix 2 = bad air pump or clogged hose
again where is pump and witch hose

A1: Air is blown behind the exhaust valves by the Secondary Air Injection (AIR) system for a maximum of 100 seconds following cold start (coolant temperature of +5 ¡C to +33 ¡C). This adds oxygen to the exhaust, initiating a secondary combustion and thereby shortening the warm-up phase of the catalytic converter. Activation is triggered by the Motronic Engine Control Module (ECM) -J220- and sent via the Secondary Air Injection (AIR) pump relay -J299- (Picture) to the Secondary Air Injection (AIR) solenoid valve -N112- (Change-over valve) and the combination valve.

I would suggest you use a stethoscope (AutoZone) to determine exactly where the noise is coming from and go from there. Odds are it is one of the valves and it is sticking. If so, you may be able to just spray it with WD-40 and be on your way.

air pump is behind bumper, rh fender, above splash panel. If you hear a loud whistle, one of the air pipes is probably broken.

Test sequence
Disconnect vacuum hose at combination valve (arrow).
Start engine and let run at idle. Vacuum must be present at vacuum hose (sense by touch) when Secondary Air Injection (AIR) pump motor is running.

If no vacuum is felt:
Switch off motor.
Check activation of valve:
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Source: OBDII.com/ubb/Forum1/HTML/002127.html

Try replacing all the vacume lines. my Grand Marquis had that problem... Source: ford-trucks.com/forums/showthread.php?t=344591

I've replaced the EGR valve, which is a diaphram (vacuum controlled) valve that lets the exhaust gas back into the intake. Next, I replaced the EGR solenoid, which is electronically controlled to open the vacuum to the EGR valve. Neither fixed my problem. A vacuum leak is a possible cause between the EGR solenoid and EGR valve, but I can't find one.

The fault could lie in a blocked EGR passage, a marginal EGR valve, or possibly a component outside the EGR system that monitors EGR rate.
The easiest way to diagnose this code is with a computer scan tool which will give you more detail than just the trouble code. With a computer scan tool the voltage signal from the DPF sensor can be monitored and compared to values in a diagnostic flow chart. DPF sensors do fail, but you want to be sure that it is bad before you spend about $100 to replace it. Source: AllData.com/techtips/2004/20040405b.html

locate the EGR DPFE sensor where it lives...remove it and tap it in your hand with the holes down. If moisture comes out, your EGR system may be plugged. Spray the inside of the sensor with computer grade solvent to get the water out. Source: CarTrackers.com/Forums/
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I got the usual CELs and went about troubleshooting the problem with VAG-COM. I discovered the 40amp fuse blown. (The books don't plainly tell you where the fuse is located), it's just outboard of the 2ndary air pump relay inside the ECM box mounted in front of the driver's side wiper. Here's what happened; the combo valve apparently has been going bad and allowed H2o to get into the air pump tubing that winds up filling up the pump because it sits at the lowest point of the air pump manifold. Because of the cold weather the water froze, causing the pump to blow the 40a fuse. The tip is: unplug the hose from the output side of the air pump everytime you change oil. this will prevent pump damage from water build up and keep you posted on the status of your combo valve too. __________________________

"In the event you have an A4 1.8T with engine code ATW with the DTC of 16795 - Secondary Air Injection system, Incorrect flow Detected, a leaking Combination valve could be the problem. A leaking valve causes exhaust gas to enter the secondary air system. The water from exhaust condensation travels through the entire secondary air injection system and if the temperature drops below 0 degrees Celsius the water will freeze. In most cases the water causes the air pump to corrode. Inspecting the combination valve will reveal the problem. If the Intake side of the valve is black, exhaust has entered the secondary air injection system."
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You may also want to consider the secondary air injection relay. It is located in the E-box in the fresh air plenum, rear driver's side of engine compartment. The factory information (CD-ROM) shows a picture of it and it is grey with "373" stamped on top. I would guess that the dealer already considered this item though.

Replacing these components looks like a pretty significant job to me. Perhaps you may want to get a second opinion from an aftermarket shop that specializes in late model Volkswagens. That's what I would do. There are couple shops in Merimack, NH, one is called EuroSport and there is another one but we can't come up with the name right now.

Hope this helps, Stan Wohlfarth
Bentley Publishers
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I have a 2000 Passat 1.8t auto that runs rough with a P0411. When I checked with a local dealer-they recommended checking for water at the secondary air pump; water was present. After confirming that, they recommended replacing the combination valve along with the secondary air pump. I checked the integrity of the hoses going to the combination (EGR/AIR) valve and they all look OK. Does anyone have any further recommendations-or should I be comfortable with just replacing these two parts? Or should the dealer be the one to work on it-though their reputation is poor, and the price quite the opposite. Thanks for any help!

If water has been in both then it might be best to replace them (both are prone to failure). VW prices the pump high, there are some sources online.

The fuse is in the ECM box, if I remember right. 40 amp.

If the pump is dead, and it is full of water, it is BECAUSE the kombi valve went bad.

The water is condensation formed by hot exhaust backfeeding into the piping down to the pump.

Quickest diagnosis is to pull the hose off at the pump. If water comes out, and it smells exhausty, order a pump and a valve.
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Links:
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Books:
Haynes VW Repair Manual

Volkswagen Passat Service Manual: 1998-2004 $120
1.8L turbo, 2.8L V6, 4.0L W8 including wagon and 4MOTION
by Bentley Publishers