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04 Cayenne S stuck in limp mode. PSM Failure

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I recently bought it from this Russian guy who calls himself a broker. I should've suspected something was wrong when her mentioned I would need a new check engine light. None the less the truck drove as it should during the test drive. Signed papers, driving home and then there's this sudden loss of power. ABS light is on, brake light is flashing, psm failure light is flashing, no mph is registering, the mileage counter isn't changing, and it seems Im stuck in one gear which has to be 4th or 5th because I can drive up to highway speeds given there's an obvious Im stuck in one gear noise coming from the trans Im assuming. I had found that pressing the e-brake while driving solved this issue until I came to a stop. There is obviously something wrong with the ebrake as well but it helped but begs the ? what would cause such an issue? Now the psm failure stays on all the time,  I can drive, slow down, stop, normally the only difference is pulling off from a stop being stuck in a higher gear I don't get the power I should at take off. 


The seller says that happened after he charged the battery, it had died due to cold weather and not being started in a while obviously. I took that had a new battery installed myself but the mechanic couldn't get his reader to read my Cayennes computer due to the psm failure. It shuts down the ecm Im thinking and that's why we can't get it to read. There are some stored codes that ultimately leads back to a failed suspension system on the truck. Could that possibly have anything to do with the psm failure and the loss of power? Any and all help is welcomed!

Edited by 955_tank
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Welcome to RennTech :welcomeani:

I'm not sure where to even start on this one.  You need to get this vehicle scanned by a Porsche specific tool to have any idea what is wrong.  Realistically, if you were a customer in my shop, I'd tell you to take it back to whoever sold it to you and get your money back; this thing could be a massive money pit.  Not to overstate the obvious, but you should never buy a used Porsche without a complete pre purchase inspection.

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  • 2 weeks later...

You might need a new engine speed sensor and may be clean your wheel speed sensors . Clear codes and see if any of them come back. 


Write down the codes before erasing just in case.

Edited by rik881
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  • 2 weeks later...

need a porsche specific scantool, like Durametric or icarsoft, but you have to hold th ebrake pedal for 30s or turn on the flashers to get the computers to wake up. 


See this post. 


Basically any weird faults, check the wiring under the carpets. This all happened the first rainy day after I bought the car, and a few days after changing the battery.

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  • 2 months later...

The Durametric tool has been a great tool for me.  


Also, I read somewhere that there are some Control Unit memories that need to be reset after taking power to zero by combinations of switching ignition on and off and doing tasks in separate combinations. Control units that get effected are:  

1. DME control unit.

2. Tire pressure monitoring system (if you have it).

3. Steering angle sensor.

4. Electric motor for sliding roof.

5. Power Windows.

6. Instrument Cluster.

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