These cars have electronic fuel management computers. It probably uses the fuel temperature as a way of determining the optimal amount of fuel to inject when all of the parameters are considered such as incoming air temp, throttle position, boost pressure, etc. Your vege conversion may be the cause of the problem after all though...perhaps it is just the sensor reading temps that don't make sense to it as it was not designed to have the fuel heated by means other than through the car's own heat exchanger. In other words it may be comparing the readings from a coolant sensor to the fuel temp sensor and knows they should be within a certain range of each other and your system is providing fuel that is outside of that expected range.

You may try putting it back to stock and seeing if the warning light/codes go away...if so you will know it's the vege conversion causing the CEL and code.

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz