P0C40: DC/DC Converter Temperature Sensor B High

P0C40 means the second temperature sensor of the DC/DC converter delivers a voltage above its valid window, which on a thermistor circuit points to a short to positive or an open line pulled up to the reference. Mercedes-Benz documents it for the EQB (series 243) and the G-Class including the electric G 580 (series 465). Information LI54.10-P-079533 describes P0C4000 as a short to positive of temperature sensor B and lists it among up to four converter codes, with P0C3B00, P0E9700 and P0E5400, that appear after the converter wakes up incorrectly. The car shows a red 12 V battery message plus HV warnings. Workshops check the converter with XENTRY, erase the memory and reset the unit before replacing anything. P0C3F is the low counterpart; sensor A uses P0C3B.

High severity. Can I keep driving?If the car still runs, drive as little as possible and go to a specialized repair shop. Do not touch the orange cables: they carry high voltage.

Symptoms

Red 12 V battery message in the instrument cluster
White or yellow high-voltage warning
12 V battery no longer charged while driving

Most likely causes

Converter entering a fault state after incorrect wake-up (Mercedes 243/465)
Open circuit or short to positive in the sensor B wiring
Internal defect of the DC/DC converter

What to check, in order

Start with the simplest. You can tick off each step.

Read the codes with a hybrid-compatible scanner and write down the freeze frame data
Check the sensor's ground and make sure the signal wire is not receiving voltage from another circuit.
Make sure the battery cooling air intake is clean
Check the 12 V auxiliary battery, which can also trigger warnings
Leave any high-voltage measurements to a specialized repair shop

Good to know

It can trigger limp mode.
It affects emissions.

What each character of P0C40 means

P: Powertrain: engine and transmission
0: Generic code: same on every make
C: Subsystem depends on the code range
40: Specific fault in that system

Frequently asked questions

Can I keep driving with code P0C40?

If the car still runs, drive as little as possible and go to a specialized repair shop. Do not touch the orange cables: they carry high voltage.

What causes code P0C40?

The most common causes are: converter entering a fault state after incorrect wake-up (Mercedes 243/465); open circuit or short to positive in the sensor B wiring; internal defect of the DC/DC converter.

Related codes

Sources

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.