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Marty, European Car Expert
Category: UK Car
Satisfied Customers: 407
Experience:  ASE & Bosch Certified Master Tech. Over 30 years experience.
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what do mean by code;P0121 and P0703 in my mercedes A clas

Customer Question

what do mean by code;P0121 and P0703 in my mercedes A class
Submitted: 7 years ago.
Category: UK Car
Expert:  Ian replied 7 years ago.
Hi Welcome to Just Answer

I will try and give you a bit of background into the explaination of these faults.

Unfortunately without any reference or details of the actual fault with your car they may or may not be relevant to the actual present concern with your car. It is not
unknown for faults to have been historical and stored for some time.
A fault code out of context can be very misleading.

P0121 Refers directly to a fault with the throttle pedal position sensor circuit A. This fault is set when the signals generated by the sensor do not match the refernce signals also generated by the pedals other sensor circuits.
These sensors have more than one sensing circuit to allow for checking of its operation.

Generally this fault is set due to wear and or a fault internal within the pedal sensor itself.

P0703 Refers directly to a fault with the circuit B brake switch , not an unusual fault code to be seen.
There are 2 brake light switches although mostly they are both inside the same casing. One switch records brake released , the other brake applied only changing when a brke application is made.
Now if due to wear or a fault the control unit sees both switches not operating in synchronisation it will set a fault code. It can also set this code due to an internal fault with the secondary circuit. This switching is used by the engine, ABS and gearbox control systems.

There is a brief description of the background of both your listed fault codes


Edited by Ian on 1/7/2010 at 10:55 PM EST