Might just be coincidence but when I left the car wash yesterday, "ESP inoperable" was on the display and the triangle with the exclamation point was illuminated. Trying to turn ESP on/off on the dash did nothing. Other than that, the car performs fine.
After reading some threads, I figured it was the steering angle sensor but since I read it could also be a myriad of other things, including the brake light switch or a speed sensor, I opted to do a little more observing (without taking it to the dealer for full diagnostic).
What I found is after restarting, I can drive the car all I want and turn the wheel either direction without triggering the warning. However upon applying the brakes for the 2nd time since starting the car, upon releasing the brake pedal, it comes on instantly. Curiously, my brake lights work fine.
Nevertheless, this puts my money on the brake light switch - anyone agree or disagree?

 

You can try the switch, it's a cheap part I think it's supposed to be around 10-15. But if you don't want to take it to the dealer to get scanned, then you should buy a scanner that can read MB specific codes, they are in the $50-$150 range and the dealer will probably charge you $150 for a diagnostic. The video on how to change the steering angle sensor is pretty accurate.

 

Also consider checking rpm sensors, see attachment.

 

Thanks everyone for the tips. I bought the brake light switch today and I'll try that first. I suspect it is the culprit since I can drive around all I want and the warning will not trigger until I press the brake for a second time and upon releasing it, the warning comes on instantly. Keeping my fingers crossed because we all know with these cars it can be anything.

 

UPDATE:
Happy to report that replacing the brake light switch fixed the "ESP inoperable" warning. The switch cost me $22 (OEM part from MBZ indy shop) and it literally took 5 minutes to replace.
Hope my weekend grease monkey diagnostics are helpful to someone else.

 

I had this happen in my car and thought it could be anything. Read this thread and noticed that my ESP light was going off after second time braking as well. Decided to gamble and buy the $21 part instead of trying to have MB diagnose. Popped the new switch in within 5 mins and voila! ESP light is gone.

Thanks!

 

I appreciate you walking this problem down to solution, and reporting your findings. I wish that this was the same symptoms I am dealing with regarding the "ESP inoperable" light on my vehicle. I just wanted you to know that I rely on things like what you wrote to help me keep mine running correctly. I appreciate it! I just purchased mine today with 114k
mileson it but there are a few little bugs I'm looking to the forum to help me sort out.

Brandon

 

FWIW, I had an identical experience...brake light switch.

relinuca

 

I'm very thankful for this thread - that's why I want to add as much meaningful info as I canmto help the next forum member out.

SPOILER ALERT: PAY SPECIAL ATTENTION TO THE WIRE/PIN COUNT ON YOUR BRAKE LIGHT SWITCH THAT'S ALREADY INSTALLED

Vehicle: 2008 E350 BASE Model w/ 140,000 miles

Rec'd a call from my mother that an "ESP Inoperable" warning popped up in center gauge.

Found this thread via Interwebs/GOOGLE Search, and asked mom if the warning popped up when she used the brakes - but she's didn't know - typical

So, I drove out and inspected her car - started the car and the warning wasn't present in the center gauge.

Because of the info in this thread, while the car still running, I pressed on the brake pedal and INSTANTLY the ESP Inoperable warning popped up - woo-hoo!

Went to FCP Euro website because over the years I've had good prices/service, entered the vehicle parameters, and it comes back saying that the car requires a 2 wire switch.

Did some price shopping (i.e. Rock Auto) and their catalog says it takes a 4 wire switch.

Didn't want to risk it, so went back to mom's house and removed the bottom panel, looked at the brake light switch that's in the car, and lo and behold it's a 4 wire switch...

Mom is a senior citizen, for the most part only putts around town now, so I saved her a few bucks and found this brake light switch with good reviews (including a 2008 E350 owner) switch for $12.99 on Amazon - here's the description.:

Olivia & Oliver Brake Light Switch 4 Pins Fits Mercedes-Benz E320 E350 E550 CLS550 Sprinter 2500 Clutch Pedal Switch for Dodge Sprinter 2500/SPRINTER 3500 Stoplight Switch

Here's the link to copy and paste into your browser - for some reason it won't hotlink on the forum!:
I ordered the switch off of Amazon, rec'd it 2 days later, and installed it yesterday.

Compared to the old switch, the looks & feel of the new switch is pretty good.

Before the new switch was installed, the warning was present, after the new switch was installed, it's off and all is well.

$12.99 in parts and 15 minutes install time with a Torx 25 bit on the end of a 1/4" bit screwdriver - (3) Torx Bolts total.

Thanks guys and I hope this helps someone out - add your experience to this tread if it does