The following is an article I wrote a few years ago about an error code I had on my 1999 Volkswagen Passat. While much of the information is model specific, it's my hope that this information well better inform you of what is likely the problem on your car. Secondary air systems are common on most new cars, and are related to the emissions system. So when there is a problem it's likely that your check engine light will turn on. It could be that a pump or valve is bad on your car, but more than likely it's just a loose vacuum hose. You'll want to check these if you can before you take it to a mechanic.

VAG Code P0411 - Secondary Air Injection System: Incorrect Flow Detected

Solution Difficulty:  Moderate

Check engine lights (CEL) can be an annoying thing. One morning you go to crank your car and the dreaded CEL comes on. The car seems to run just fine, but under emission guidelines, your car is not OK. To make matters worse, you won't be able to pass inspection until the problem is fixed and the light is out.

The problem with my Passat started back around Thanksgiving '03. One cold day, I started the car up and the CEL came on. The first thought through my mind was "oh no, not again". You see, I had just fixed the Bug which was suffering from a bad catalytic converter. Luckily, the folks at Volkswagen fixed the problem under the emissions warranty, but as I was later to find out this would not always be the case.

I had scanned the car with VAG-COM and determined that the problem was with the secondary air injection system, but for whatever reason I let time go by and didn't fix it immediately. When I realized that, my state inspection was coming up and I decided to fix the problem. So I called up the local VW service department and asked if the secondary air system was covered. As I expected it was not. The only parts that are covered under the 77,000 mile warranty are the Mass Air Flow Sensor (MAF) and the catalytic converter. I was on my own.

After a little research, I discovered that several people had encountered problems with the secondary air system. According to the Bentley manual, the secondary air system is an emission related system that injects extra oxygen into the exhaust pipe to burn off any extra fuel. The system only activates on cold starts (when the coolant temperature is between 15 and 35 degrees Celsius). The system consists of a large air pump, one or two combi-valves and a solenoid. The valves are controlled via vacuum lines that cause them to open and close.

Several of the folks who've had problems with secondary air stated that the vacuum lines were often the chief cause of malfunction. I knew from the error message that air was not being injected into the tailpipe. So, with my Bentley manual in hand (on computer actually), I cranked up VAG-COM and started to conduct the output tests for the engine module. Two of these tests cause the relays to switch off and on, which allows you to do a vacuum test. Another test actually turns on the air injector motor, which lets you verify that the motor is actually working.

I quickly eliminated all these as cause, so I moved underneath the car to check the combi valves. I wasn't sure how I'd check them without removing them, but as luck would have it, I didn't need to remove them at all.

Because this car has two exhaust banks, it also has two combi valves. The vacuum line to the driver side travels across the engine in a solid plastic tube, which I suspected may be cracked. I did a vacuum test and found this not to be the case. I then checked the flexible, shielded pipe connecting the plastic tube to the combi valve. Still no leak, so I proceeded to the passenger side.

I would like to note that these two valves are in tight quarters, and the manual actually recommends you remove the coolant reservoir to access the valve. I found this unnecessary since I could easily get to both valves from beneath the car. When I checked the combi valve on the passenger side, I found that the vacuum line was badly damaged, and barely attached. (There may have been a recall for this) Undoubtedly the vacuum leaked caused by this bad hose was causing neither valves to open, producing the "Incorrect Flow" fault that VAG-COM detected.

Since there wasn't enough spare tubing to cut off the frayed end, I spliced another vacuum line off the Y connector and into the remaining bit of shielded line. I connected this line back to the combi valve figuring that its high temperature resistance would be better than an entirely new piece of rubber tubing. Then I cleared the codes from the computer and restarted the engine.

At this point, I needed to let the car run for a bit and conduct the self diagnostics. In my case, I took the car for a test drive. Luckily, the CEL has not returned, and the repair didn't really cost me anything since I had the piece of vacuum tubing lying around. Of course, I am in the process of ordering a new piece of tubing and I will replace my temporary fix the next chance I get.

The way I figured it, I save a lot of cash on this one. Secondary air pumps are about $300, and I have no idea how much the relays or combi valves run. In all I spent a couple of hours on the repair, so I would estimate that would costs a couple hundred dollars in labor. In reality, you wouldn't even need to jack the car or put it on ramps. You could reach either valve by removing the coolant reservoir tank or part of the intake plenum.

This article contributed by ERSWeb
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