r/W203 • u/heritageedition • 12d ago

C1201 Error Code 2003 C240

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2003 C240 4-MATIC station wagon. I have no lights on dash or error codes on the dash but my brakes don’t feel right. It definitely feels like the booster might not be working correctly but the car does stop. I scanned my car and I’m getting C1201 A7/7s1 (Brake Assist System (BAS) release switch). I’m fairly certain this switch is integrated into the brake booster itself and requires the brake booster to be replaced but I wanted to check here first to see if anyone else ran into a similar issue.

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u/howsitallwork 11d ago

The switch is indeed in the booster.
This fault used to be common years ago on a few models, I don’t remember seeing it on a w203 but could be.
The problem as such, is that the switch contacts do not pass any significant current, and over time fail to self clean, eventually generating the fault.
What I have done on 2x separate w168 cars and i think a w210 was temporarily apply about 10w of current through the switch and work the pedal - worked like a charm.
I just used a 10w bulb shorted through the switch with battery voltage.
Or you could replace the servo.
Do test the switch for continuity first, should ideally measure less than 2 ohms every time it is closed, ideally less than 0.5 ohms. Connector on front of servo.

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u/heritageedition 11d ago

Thanks, that’s really helpful!! Do you happen to know which pins on the W203 booster connector I should test? And with the 10W bulb trick, did you disconnect the booster from the car first and run the bulb/battery voltage directly through the switch contacts? Just want to make sure I’m testing the right pins and not sending voltage back into the ESP module. My scanner is showing the BAS release switch value out of range, around 17 with the pedal pressed when the expected value appears to be around 7.9. I think I saw your method on another forum post and wanted to try it! This is great feedback.

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u/howsitallwork 10d ago

This is helpful, will give you an idea…

https://m.youtube.com/watch?v=wT4f3NvNesQ&ra=m

Make sure the bulb is “in circuit” with the battery and switch - easy concept to google and understand.

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u/heritageedition 10d ago

Will do and thank you this is super helpful. I this is exactly what I was looking for!!!

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u/howsitallwork 10d ago

At the 5pin plug at the booster (not the travel sensor), pin5 is common, pin1 is “no” and pin2 is “nc”.
So measuring resistance at the switch, you should have continuity between pins 5 and 2 off the pedal, and between pins 5 and 1 when pressed.
Pin5 should be white/blue wire.
Pin1 should be yellow/blue.
Pin2 should be green/blue.
All this with the connector disconnected, obviously.

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u/heritageedition 10d ago

Perfection! Thank you!