BMW 2ACC — DME Master Relay Shift Delay

Severity: Informational
Module: DME
OBD-II Code: P16C5

Description

The DME master relay (also called the DME main relay) supplies switched 12 V power to the DME and to power-hungry engine loads downstream of the DME, including the fuel injectors, ignition coils, and various sensors. The DME monitors the relay's response time after terminal 15 (ignition on) and expects the relay contacts to close within a defined window. When the relay is slow to switch in, this code is stored.

The DME measured a switching delay greater than the allowed threshold (approximately 110 ms per the ISTA monitoring definition) when waking the main relay. This is a hardware fault on the relay, its control circuit, or the supply feeding it.

Safety Warnings

A failing main relay can cause intermittent crank-no-start conditions that may strand the vehicle. If symptoms include occasional no-start events, address this fault before driving in situations where reliable starting is critical.

2ACC - P16C5: DME Master Relay Shift Delay - Main Relay Switching Delay

Symptoms

Most vehicles display no drivability symptoms when this code sets alone. Possible symptoms as the fault progresses:

Intermittent crank-no-start (engine cranks but does not fire)
Hard or prolonged start, especially after the car has sat overnight
Random no-starts that self-clear after the key is cycled
Secondary fault codes for sensor supply voltage, throttle potentiometer, or pedal position sensor — these appear because the relay's delayed response starves those components of power

Common Causes

In rough order of frequency:

Aging DME main relay with pitted or high-resistance contacts
Corrosion on the relay socket terminals
Loose or corroded ground on the relay control circuit
Low battery voltage during the switch-on event (weak battery or poor battery terminal connection)
Damaged wiring between the relay and the DME
Internal DME fault (uncommon; rule out the relay and wiring first)

Diagnosis Steps

Monitoring condition per ISTA: battery voltage > 10 V, terminal 15 on.

Check battery resting voltage (12.4 V or higher) and voltage during cranking (above 10 V). A weak battery alone can cause this fault to set spuriously.
Locate the DME main relay. Position varies by chassis and generation — on many E-series it is in the front power distribution box (E-box) near the cabin filter; on F-series it is commonly on the front right side of the engine bay fuse panel. Consult a wiring diagram for your specific vehicle.
Pull the relay. Inspect the socket pins for corrosion, heat discoloration, or backed-out terminals.
Test the relay coil resistance and listen for a clean click when 12 V is applied to the coil. Swap with a known-good identical relay as a diagnostic step.
Check the 30 A fuse that feeds the relay's input (terminal 30). Fuse location varies by model.
Check continuity and voltage drop across the relay control circuit from the DME.
Clear the fault after any repair and drive through several key-off sleep cycles so the monitor can re-run.

Resolution

The most common fix is a new DME main relay, with the relay socket cleaned or re-tensioned as needed. Confirm the correct BMW part number for your vehicle via RealOEM using the VIN before ordering.

If the relay and wiring test good, the next likely causes are a weak battery or a poor ground. Replace the battery if it fails a load test and register the new battery via ISTA or an equivalent tool.

DME internal failure is the last item to rule out. Do not replace the DME based on this code alone.

Module Reference: DME