http://jalopnik.com/5066115/new-benz-bluetec-diesels-wont-run-without-urea

To meet stringent new diesel emissions criteria here in the States, new Mercedes Benz BlueTec diesels use urea, carried in a separate tank, injected into the catalyst. The urea must be replenished at service intervals — generally 10,000 miles — and now Mercedes acknowledges the vehicles won't start if the urea is allowed to run out. BlueTec cars and SUVs will notify the driver when the seven-gallon urea tank reaches one gallon, and again when it runs completely out, allowing 20 additional starts before disabling the vehicle. At least two gallons of fluid must be added for the system to reset and permit the car to be started. Now, where would you find two gallons of urea on the open road?

Mercedes dealers will sell urea in half-gallon containers, using a special mess-free store-and-fill bottle, for $7.50, but that's the expensive option. At service intervals, using bulk containers, Mercedes is expecting the fluid to run no more than the cost of diesel fuel. If you run out of emissions juice, urea transport and refill is reportedly included in your roadside assistance plan, as is tire changing — fortunate since the urea tank occupies the spare tire well. [Automotive News, Sub. Req.]

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If it were me, I'd stick the sensor that detects urea content into a separate urea jar and just fill the main tank with plain water. Problem solved.