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Home >> Audi >> 2007 >> Q7 4.2 >> Repair and Diagnosis >> Engine Performance >> System >> 4.2L - Generic Scan Tool - Engine Code: Bar (General Information) >> St Generic Scan Tool >> Description And Operation >> SCR NOX Catalyst System >> Reducing Agent Heater Control Module -J891-
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Reducing Agent Heater Control Module -J891-

The SCR system contains a heater for the tank and the dosing module, including the supply line, since the reductant freezes at temperatures below 11 degrees below zero Celsius (12 degrees Fahrenheit). The electric resistance heater thaws the reductant so that it is ready for dosing. The SCR dosing line heater circuit has a high-side driver and a low-side driver, the current of which can be monitored separately. The advantage of this is that, when a short circuit is detected in one, the other driver can be switched off to ensure component protection. The SCR heater additionally has pull-up and pull-down resistors, which are used to monitor the lines when the heater is not active. For this purpose, a transistor is used to switch defined voltage levels via high Ohmic resistors (pull-up, pull-down) and the voltage of the corresponding lines is monitored. The circuit monitoring of the SCR dosing line heater takes place in the SCR control module, which transmits the fault messages to the ECM, where the corresponding fault is stored in the fault memory.

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