X350 p0171 p0174 p0491 codes possible causes?

(gcoder1) #1

On my 2006 XJ8L I suddenly got these 3 codes today. The check engine light came on within the first 20 minutes of driving. The car still ran perfectly for the remainder of the journey. If it were not for the illuminated check engine light I would not have known anything was wrong.
P0491 Secondary Air Injection System
P0174 Bank 2 system too lean
P0171 Bank 1 system too lean

Are all 3 of these related to the same cause? Any suggestions as to what to check?
Thank you for your knowledge.

(Paul Wigton) #2

Input the trouble codes in your browser: Id be somewhat surprised if there isnt some relevant info offered up.

(gcoder1) #3

I did some research on the codes and found several possible causes of theP0491 and started checking things out. One common problem seemed to be a broken vacuum line in the system.I located the components of the system and found that the vacuum went to a storage tank before it ended at the Air Control Valve which is located in the front of the engine below the upper radiator hose (slightly on the right side). I pulled the vacuum hose off and connected a line that I could blow through. With the key off the air went out to atmosphere as it should. With the engine running There was good vacuum on that hose so it didn’t appear to be a broken line. I then hooked up a hose for me to apply vacuum to the Air control valve and when I applied vacuum I could here that the valve was opening to the exhaust and shutting when vacuum was released. I did that a few times just to confirm. At that point everything was pointing to the Supplemental air pump. I checked the fuse which was fine. I shut the car off and attached the vacuum line as it was supposed to be. After I started the car I was confident that I could hear the pump come on.
I was stunned that I could not find the cause so I attached my code reader to check the codes and clear them just to see if they would come back. To my surprise the P0491 had cleared itself. I went ahead and cleared the other 2 codes. It is such a crappy weather day here that I didn’t go for a long drive, but for the short, 6 miles, drive there were no codes. I am not ready to declare victory yet though.