No boost on 1.8t AWM - 17964/P1556 - Charge Pressure Control: Negative Deviation
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  1. #1
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    No boost on 1.8t AWM - 17964/P1556 - Charge Pressure Control: Negative Deviation

    Have had success on here in the past, and was hoping someone could give me some more specific advice than what I've found so far though a ton of googling.

    Currently I am experiencing a loss of boost in acceleration, not hearing that all familiar pssss when I hit the gas. The code i'm getting, 17964/P1556 - Charge Pressure Control: Negative Deviation on the ross-tech wiki shows some possible causes.
    17964/P1556/005462 - Ross-Tech Wiki

    There's a bit of a back story here that may help someone knowledgeable: faulty MAF sensor and bad coil pack led to a flashing CEL. Pulled over and tried to identity which one was bad, tried swapping them around and had no luck getting the misfire to stop. I did notice a little bit of white gunky stuff on the coil, no clue what that was and never was able to find an explanation. But since then I've put in a new MAF, spark plugs, and all new coil packs on. That got rid of all those related codes on the vagcom and no more misfire.

    The major problem was this happened about 2am and I was 20 minutes from home. So, I didnt really know what else to do but try and make it that 20 minutes with a flashing CEL. When I got home in the drive way, I popped open the hood to see some white smoke and the catalytic converter glowing red. Other than no boost, the car now drives fine, and I only get the one code listed above, but I know driving with a flashing CEL isn't smart and may have cause some permanent damage.

    My preliminary guesses are a clogged cat, failed n75, or problem with the wastegate. Havent been able to find any problem with vacuum lines yet, but I could have missed a few spots. The fact that power loss happened halfway home after driving with the flashing CEL makes me think the cat convertor makes the most sense.

    Any tips on how and where to start troubleshooting?

    I did disconnect the n75 sensor and i dont know whether it should or not, but there was no noticeable difference in the car. I just got an error code saying ther was an open circuit. Afterwards got an ohm reading of 30 on it. I was also attempting to squeeze my hand and find the wastegate actuator rod I've read about, but I was either unable to reach it or wasnt looking in the right spot. Never felt anything like I've seen in other pictures.

    Thanks a ton to anyone who may have some input here.

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  3. #2
    Overdrive Steve in Chicago's Avatar
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    I agree with your primary suspect being a damaged CAT. If you have access to a bore-scope, you can try pulling the front O2 sensor and inspecting the CAT core. Or, you can disconnect the CAT from the turbo and inspect that way (more work).

    My original CAT core collapsed and large chunks of it tumbled down the exhaust path blocking it. The car started, idled and drove normally as long as you didn't ask the engine to breathe much. As soon as you tried to get any power from it and the exhaust flow increased, it just had no pull. I was also getting a Charge Pressure Control (don't remember if it was Positive or Negative) Deviation code. I finally noticed the exhaust was making an air-wooshing noise when I was deep into the throttle and the RPM's were high. I looked up the tail pipe with a flashlight and saw the big chunks of core blocking flow. My exhaust is the TT 2.5" with Borla mufflers, so the chunks were able to make it all the way back to the tail pipe where I could spot them. If you have a stock system, any chunks will likely get trapped in the resonator. I was also lucky that my exhaust was only a couple years old and stainless, so I was able to dismantle it to clear it. If you're working with the original exhaust, this might be difficult.

    Find a way to inspect the CAT core. Be aware, a blocked exhaust will cause the exhaust valves to run HOT. Too much of this will damage the exhaust valves.

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    Thanks for the advice Steve, I'll see what I can do. My car is all stock, so it sounds like I'll be investigating things the hard way!

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