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I have a 06 SL500. On Thursday of last week I took my car in to get it smogged. That morning when I turned the key on I got a red message on the speedometer saying "Brake" and another red message under the tach saying "Reduced brake effect start engine" (I also got the same messages this morning). When I started the car the message went away and I drove into town to the SMOG place. The car passed the test and I left. After I left the SMOG shop within a quarter of a mile I noticed the car wasn't upshifting and I had a yellow message saying, "Check Engine." I pulled over and tried different things and nothing seemed to help the problem so I drove home slowly. When I got home I threw my code reader on the car and got the code P0798. I also noticed when I got home that shifting the car into forward or reverse from park was jerky/snappy.

Again, the symptoms I'm experiencing are the transmission stays in 1st gear no matter how fast I go and only first, reverse and park work. The check engine light is on as well and I’ve gotten that Brake Message twice (once today). Additionally, the car jerks when I put it in drive or reverse.

I am certain the SMOG shop didn't do anything that could have caused the problem. The only think the SMOG guy did was remove the front engine cover.

In looking up the code (P0798), while there was a list of five things, it said the most probable cause of the code was the pressure control solenoid valve circuit is open or shorted. However, I don't understand why I'm getting the red brake message and in talking to the owner of a small Mercedes Shop in town he said it might be something like a circuit board or some other electronic device.

Has anyone ever experienced this kind of thing?

Thanks.
 

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2004 sl500 Pewter Metallic
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I had same thing on my 2004 sl500. turned out to be a valve body control module. 1100.00 at dealer
 
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