Why does my BMW start only on the second attempt with 8040B5?

BMW fault 8040B5 with a car that sometimes starts only on the second attempt means the failed start needs to be captured under load. I would test battery voltage, the high-current cables, grounds, starter-control command and starter response during the failed attempt before ordering a starter.

Why does my BMW start only on the second attempt with 8040B5?

An F30 owner described an intermittent first-attempt failure and stored fault 8040B5, shown as an engine-start fault during starter operation with no engine speed detected on activation. The car would then start on the second attempt. Other undervoltage faults were present in the history, although the owner measured about 12.9 volts with the engine off and did not notice normal battery symptoms.

That resting measurement does not settle the diagnosis. Starting is the battery's heaviest routine load. A battery, cable or connection can look normal with almost no current flowing and then lose too much voltage when the starter is requested.

The code also does not identify the failed part. It records the result: a start request occurred, but the expected engine-speed response was absent. The diagnostic job is to find whether the starter was commanded, whether enough electrical power reached it, and whether it turned the engine.

Classify the failed attempt by sound and data

The first failed attempt should go into one of three groups.

• Silent: no click and no cranking sound
• Click without cranking: the solenoid may engage, but the starter does not turn the engine
• Slow or irregular crank: the engine turns, but speed is weak or inconsistent

Each pattern changes the next test. Silence puts more attention on the control request and interlocks. A solid click with no rotation raises the high-current path and starter. Slow cranking makes voltage drop, ground integrity, battery condition, mechanical load and starter condition important.

Record the exact sequence. Did the lights dim? Did the starter click once or chatter? Did the car wake normally? Was the engine hot or cold? Did Auto Start Stop operate before the first failure? Those observations are not a diagnosis, but they help reproduce the right event.

Save the complete fault record before clearing it. Keep fault frequency, mileage, terminal status and voltage data. Historical undervoltage faults can be useful even when they appeared months earlier, but their age and frequency matter.

Measure voltage while the fault is happening

A useful starter diagnosis needs loaded measurements. Start with the battery posts, not the cable clamps. Capture voltage from before the start button is pressed through the failed attempt and the successful attempt.

Then measure voltage drop across each side of the high-current circuit while the starter is loaded:

1. Battery positive post to the starter's main positive connection.
2. Starter housing or engine block to the battery negative post.
3. Any major connection or ground strap that shows excessive loss.

Voltage-drop testing asks how much electrical pressure is lost across a connection while current flows. It can expose corrosion, loose fasteners, damaged cables or a failing ground strap that a static continuity check misses.

Use the vehicle's wiring information and safe access procedure. The starter circuit carries high current. Avoid shorting a test lead to ground, and do not work near moving components during a remote start request.

A battery conductance or load test also belongs here. Resting voltage can be normal after charging or a recent drive even when available starting current is poor. Compare the first and second attempts. If voltage stays steady but the starter does nothing despite a confirmed command, the starter branch becomes stronger. If voltage collapses or a cable consumes a large share of the available voltage, repair the supply path first.

Confirm command before blaming the starter

BMW's F30 powertrain training describes the vehicle's start-control architecture, while BMW's diagnosis material stresses testing the system rather than replacing the component named by a symptom. For this fault, confirm whether the starter-control circuit receives a request during the failed attempt.

The exact terminal names and access points depend on the vehicle and engine. Use VIN-specific wiring information. The useful logic is consistent:

• Was a valid start request recognized?
• Did the control side command the starter?
• Did the command reach the starter solenoid?
• Did the high-current circuit support the load?
• Did the crankshaft begin turning?

A missing command keeps the diagnosis on the authorization and control side. A present command with healthy supply and ground, followed by no rotation, points more directly toward the starter or its solenoid. A command that arrives while supply voltage falls sharply keeps battery and connection faults in play.

BMW service information for older intermittent no-crank complaints instructs technicians to reproduce the condition, preserve fault memory and follow the start-function test path rather than deleting evidence first. Its model-specific steering-lock details do not apply to every F3X, but the method is sound: duplicate, record, test the start chain, then repair.

Avoid the common false conclusions

A normal alternator voltage does not prove the battery, cable and starter are healthy. Charging voltage is measured after the engine is running, which is outside the failed event.

A successful second attempt does not prove the starter is healthy. Worn contacts and temperature-sensitive connections can work intermittently. The useful question is what changed between attempts.

A collection of old undervoltage faults does not automatically mean the battery is the root cause. Check their dates, frequencies and modules. A single past low-voltage event differs from several modules recording a voltage drop during the same failed start.

Replacing the starter may still be correct, but the evidence should support it: a confirmed command, acceptable battery behavior, low voltage loss in the cables and grounds, and no starter rotation. Without that chain, a starter replacement remains a guess.

Prove the repair under the original conditions

After repair, repeat the same cold or hot start condition that produced the fault. Record several starts, not one. Confirm that cranking is prompt, the voltage trace is consistent and 8040B5 does not return.

If the failure cannot be reproduced, preserve the logger setup and wait for a clean capture rather than buying parts from the fault text. An intermittent start problem often becomes diagnosable when the failed and successful attempts are placed side by side.

For an F3X that starts on the second attempt, the shortest path is a synchronized record: command, battery voltage, cable voltage drop, starter sound and engine-speed response. That record separates a weak supply from a missing command and a starter that received everything it needed but still did not turn.