Engine Light ON (or Service Engine Soon Warning Light),Excessive Fuel Consumption
P0172 MERCEDES-BENZ Possible Causes
Faulty front heated oxygen sensor,Ignition misfiring,Faulty fuel injectors,Exhaust gas leaks,Incorrect fuel pressure,Faulty Mass Air Flow (MAF) sensor
P0172 MERCEDES-BENZ Description
This fault code is stored in the Engine Control Module (ECM) when the ECM is no longer capable of controlling the air fuel ratio. For correct operation of the catalytic converter the ECM is required to keep the air fuel ratio (AFR) at approximately lambda 1, this equates to an AFR of 14.7:1 by mass. This is achieved by trimming the fuel mixture to the required amount to cater for slight anomalies during normal operating conditions. However when a fault causes the fuel trim to exceed a predetermined level the malfunction indication light is illuminated and a limited operating strategy is employed. The pre-catalyst exhaust gas oxygen sensor is the primary component used for calculating the fuel trim once the system enters closed loop control. The fuel trim data can be displayed as a percentage, either positive or negative. Under this condition the fuel trim data will display a negative value. The ECM will decrease the injection duration to counteract the excessively rich mixture. This fault can be stored due to a faulty coolant temperature sensor, excessively high fuel pressure or a faulty fuel injector. The fuel trim is monitored under various loads and conditions. If a load or condition is included in the fault code this can be an advantage during diagnosis. Normally a code set during idle or light load is relating to an air leak. And a code set during high or part load can be a fuelling issue or an erroneous sensor signal.
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