Failed K40 relay module (most common).; Blown fuse protecting the K40 relay circuit.; Corroded or loose electrical connections at the relay, fuse box, or engine control module.; Damaged wiring harness between the K40 relay and the engine control module.; Faulty engine control module (rare).; Water intrusion into the relay/fuse box causing short circuits.

DTC codes / Mercedes-Benz codes / P1610

P1610 · Mercedes-BenzVoltage Supply Missing Or Relay Module K40

Q: What does code P1610 mean?

Voltage Supply Missing Or Relay Module K40. P1610 on Mercedes-Benz vehicles indicates that the engine control module has detected a missing voltage supply to the K40 relay module, which is the main power distribution relay for the engine management system. The computer expects a stable voltage on the K40 control circuit and sets this code when that voltage is absent or out of range, often due to a faulty relay, blown fuse, or wiring issue.

P1610 on Mercedes-Benz vehicles indicates that the engine control module has detected a missing voltage supply to the K40 relay module, which is the main power distribution relay for the engine management system. The computer expects a stable voltage on the K40 control circuit and sets this code when that voltage is absent or out of range, often due to a faulty relay, blown fuse, or wiring issue.

SystemPowertrain

Code typeMercedes-Benz specific

SeverityMedium

Fix difficultyIntermediate

AreaComputer and output circuits

Symptoms you may notice

Illuminated check engine light or instrument cluster warning message.
Engine may crank but not start, or stall intermittently while driving.
Multiple electrical accessories (e.g., radio, climate control) may malfunction if the K40 relay also supplies them.
No noticeable performance issues besides the warning light in some cases.
Rough running or reduced power if the voltage supply to the engine control module is intermittent.

Common causes, most likely first

Failed K40 relay module (most common).
Blown fuse protecting the K40 relay circuit.
Corroded or loose electrical connections at the relay, fuse box, or engine control module.
Damaged wiring harness between the K40 relay and the engine control module.
Faulty engine control module (rare).
Water intrusion into the relay/fuse box causing short circuits.

How to diagnose and fix it

Check all fuses related to the K40 relay and engine management system; replace any blown fuses.
Inspect the K40 relay for signs of burning, corrosion, or loose pins; swap with a known-good relay of the same part number for testing.
Measure voltage at the K40 relay socket with the ignition on; compare with battery voltage to identify a supply issue.
Check wiring continuity and resistance between the K40 relay and the engine control module, looking for opens or shorts.
Inspect the relay/fuse box for water damage or corrosion and clean or repair as needed.
If all electrical checks pass, test the engine control module for internal faults, but only after ruling out all other causes.

K40 Relay Failure Patterns and Diagnostic Traps

The K40 relay often fails intermittently, so the code may not be present when the vehicle is tested; a wiggle test of the relay and wiring while monitoring live data can reveal the fault.
A weak battery or poor ground connection can mimic a failed K40 relay by causing voltage drops that set P1610, so always load-test the battery and check grounds first.
On some Mercedes-Benz models, the K40 relay also powers the secondary air injection pump and other emissions components, so a failure can trigger additional codes like P0410.
Replacing the K40 relay without checking the fuse and wiring is a common misdiagnosis; always verify power and ground at the relay socket before condemning the part.
The code may be stored in the engine control module but actually caused by a faulty overvoltage protection relay or a failing alternator, so check charging system voltage under load.

Can I keep driving with P1610?

If the engine starts and runs normally, you can drive the vehicle to a repair shop, but avoid long trips or heavy electrical loads. If the engine stalls or fails to start, do not drive and have the vehicle towed to prevent safety risks.

Tools that help

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Guidance is general and written for Mercedes-Benz vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.