29D9

What the Code Means

29D9 logs a misfire in cylinder 5. The DME registers that the power stroke in this cylinder isn't happening or is unstable, and stores the code. The fault is typical of BMW six-cylinder engines. In most cases the cause is in the ignition components — the coil or plug for that specific cylinder. If the coil and plug are ruled out by swapping them to another cylinder, the likely culprit is a failed ignition driver in the DME: the output stage that fires the cylinder 5 coil either isn't sending a signal or is sending a defective one.

Symptoms

• Rough idle, engine shaking at idle
• Loss of power
• Hesitation and stumbling under load
• Check engine light on

A single-cylinder misfire is most noticeable at idle: the engine runs unevenly and vibrates because one of the six power strokes isn't delivering torque.

Diagnosis and Repair

Step 1. Swap the Ignition Components

Swap the coil and plug from cylinder 5 to another cylinder. This is the basic test that splits the cause into two branches:

• The misfire moves with the coil and plug — the ignition components are faulty, replace them.
• The misfire stays on cylinder 5 — the components are ruled out, and the DME is suspect.

Step 2. Check the Control Module

With the coil and plug ruled out, bench-test the DME's ignition outputs. The goal is to confirm that the cylinder 5 ignition driver isn't producing a control signal.

Step 3. Repair or Replace the DME

Based on the bench test results, the ignition driver circuits in the DME are repaired or replaced. This is the most complex and expensive fix — only go there after the ignition components have been reliably ruled out by swapping.

Secondary Damage

Unburned fuel from the misfiring cylinder enters the exhaust, and this isn't a repair you want to put off:

• Catalytic converter — the unburned mixture burns off inside the converter and destroys its honeycomb.
• Oxygen sensor — gets contaminated by products of incomplete combustion and starts reading skewed.
• Fuel economy — drops because the engine is running with a dead cylinder and a rich mixture.