Chassis, generic

C05CA Brake Master Cylinder Piston Position Sensor A Circuit Voltage High

Check engine lightOn
EmissionsNot relevant
Drive cycleNot required
Limp modePossible

Channel A of the travel sensor at the master cylinder input rod reports a signal beyond its upper limit. In GM's booster-type brake modules this channel is a linear position sensor (LiPS) that talks to the EBCM digitally over SENT, so the high error is a message from the sensor itself: the Cadillac XT4 table in 19OBDG03D reacts within 0.033 s on the fast channel, 23OBDG07 and 25OBDG03B after 0.96 s on the slow channel, Type A with MIL. Line shorts and lost messages are filed under separate network codes there. Continental describes pedal modules that read travel with two sensors so one bad channel can be identified. Check the sensor connector and supply before replacing the booster assembly. C05CB is the low counterpart.

Likely causes

Position sensor A reporting out-of-range high internally
Supply voltage of the sensor out of tolerance
Sensor magnet or input rod out of position

Symptoms

  • Malfunction indicator lamp plus brake warning on GM models
  • Reduced brake assist or changed pedal feel

Repair estimate

DifficultyHardWorkshop job
Parts and labour€100–2,000
Time0.5–3 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

master cylinder pos sensor aelectric brake booster

Related codes

C05CBC05CCC05CDC05D1

References: gsi.ext.gm.com, gsi.ext.gm.com, gsi.ext.gm.com, patents.google.com.