2014 BMW 328D Won't Start Due to P0685: Power ECM Relay Location and Diagnosis

Owning a car is great–right up until the day it decides it doesn’t feel like starting. If you drive a 2014 BMW 328D and you’re staring at a no-start situation with a code like P0685, you’re not alone. That code is basically your BMW’s way of saying, “Hey, the engine computer isn’t getting the power it needs.” And without that power, nothing else really gets a chance to work.

At the center of it all is the Engine Control Module (ECM)–the “brain” that manages fuel delivery, timing, emissions, and a long list of behind-the-scenes decisions that keep the engine running smoothly. But the ECM can’t do any of that if it can’t wake up. That’s where the power ECM relay comes in.

What’s Actually Happening When This Goes Wrong

When you turn the key (or press the start button), the car expects the power relay to click on and send voltage to the ECM. Think of the relay as a gatekeeper: ignition turns on → relay activates → ECM gets power → the engine management system comes alive.

If that chain breaks anywhere–bad relay, weak voltage, damaged wiring–the ECM stays dark. And when the ECM is offline, the car can crank and still not start… or it may not even try at all, depending on what the system is seeing.

The P0685 trouble code shows up when the vehicle detects that the ECM isn’t receiving the correct power feed. It’s simple in theory, but in real life it can send people chasing the wrong parts.

What Usually Causes P0685 in the Real World

Most of the time, this isn’t some mysterious “BMW-only” nightmare. It’s usually one of a few common culprits:

• A failing relay: Relays wear out. Heat, age, and electrical load add up over time. Once the relay gets flaky, power to the ECM becomes unreliable–or disappears completely.
• Wiring or connection problems: Corrosion, moisture intrusion, rubbed-through insulation, or a loose connector can interrupt the power path between the battery, relay, and ECM.
• Battery or voltage issues: A weak battery can create low-voltage conditions that look like relay failure. This gets overlooked a lot because people assume “the battery seems fine.”
• A faulty ECM (less common): Yes, it can happen–but it’s usually the last thing you want to blame until everything else is confirmed.

How a Technician Typically Diagnoses It (Without Guessing)

Pros don’t jump straight to replacing the ECM, because that’s expensive and often unnecessary. Instead, they work the problem logically:

1. Confirm the code with a scan tool and check for related codes that might add context.
2. Verify power and ground at the relay and at the ECM–usually with a multimeter, not assumptions.
3. Test the relay (often by swapping with a known-good one, if an identical relay exists in the box).
4. Inspect wiring and connectors for corrosion, damage, or looseness.
5. Check battery health and connections because low voltage can cause misleading symptoms.

Good techs also lean heavily on BMW-specific wiring diagrams and service info, because guessing wire colors and circuits on modern cars is a fast way to waste hours.

Common Missteps People Make

One of the biggest mistakes is assuming P0685 automatically means “replace the ECM.” It *can* be the ECM, but it’s far more often a relay, wiring issue, or voltage problem.

Another common trap: replacing the relay without confirming it’s actually not being powered correctly–or without checking whether the issue is upstream (like a battery connection) or downstream (like wiring to the ECM).

Tools and Parts That Usually Come Into Play

To diagnose and fix this properly, you’re typically looking at:

• A scan tool/code reader
• A multimeter (for voltage and continuity checks)
• A replacement relay (if testing confirms it’s failing)
• Possible wiring repair supplies/connectors
• A battery tester (or at least a solid voltage/load test)

Bottom Line

On a 2014 BMW 328D, P0685 points to an ECM power supply problem, and that can absolutely lead to a no-start situation. The power ECM relay is usually located in the under-hood fuse/relay box, and it’s one of the first things worth checking–right alongside battery condition and wiring integrity.

Handle it step-by-step, and you’ll avoid the most expensive mistake people make with this code: replacing parts before you’ve proven they’re actually bad.