BMW Won’t Start: Causes and a Quick Check
Introduction
A familiar scene: you turn the key and the engine doesn’t catch. The starter cranks in vain, only a relay clicks, or nothing happens at all. “Won’t start” isn’t one fault—it’s a chain of conditions that have to line up in order.
Break any one link and the engine won’t start, even if every other system is healthy. That’s why the right approach isn’t swapping parts on a hunch—it’s walking the chain in sequence and finding the real break.
Why “Won’t Start” Is Always a Chain
An engine starts only when several conditions are met at once. The electrical system needs adequate voltage: below 12.2 V, the starter often still cranks, but the engine won’t fire—the control unit lacks the power for stable injection and ignition.
Next comes the electronic safeguard. The EWS/CAS immobilizer has to authorize the start—otherwise the ignition switches on, but there’s no injection. The remaining links: the starter signal through terminal 50, the RPM signal from the crankshaft sensor, fuel pressure from the pump and the high-pressure pump (HDP), spark, and compression.
You work through the diagnostic chain in exactly this order:
voltage → EWS/CAS → starter signal → RPM signal → fuel → ignition → compression.
Swapping the steps around is pointless: each stage depends on the one before it being sound.
The Most Likely Causes
| Cause | How It Shows Up | What to Check |
|---|---|---|
| Battery voltage below spec | Starter cranks, engine won’t fire | Voltage at rest and under load |
| Immobilizer EWS/CAS | Ignition switches on, no injection | A second key, EWS/CAS status |
| Starter motor | No cranking sound, just a click | Terminal 50, ground, the starter motor itself |
| Crankshaft sensor | Engine cranks evenly but won’t start; 2A99 fault | Code in the DME, RPM signal |
| No fuel pressure | Starter cranks, no start | Fuel pump, relay, HDP |
| DME relay / ignition switch | No-start | More common on E-series than on F/G-series |
The 5-Minute Quick Check
- Measure battery voltage: at rest—above 12.4 V; under load—above 10 V.
- Test with a second key (to rule out EWS/CAS).
- Check whether the starter motor actually cranks. If not—check terminal 50.
- Pull the fault memory—not only from the DME, but from the CAS as well.
- Measure fuel pressure at the fuel rail.
Diagnostic Equipment
The first meaningful step is reading the fault memory and the live data stream. Only then can you judge which link in the chain is affected. Until you have codes, any verdict about a specific part is premature.
For this stage, three tools that fit, listed neutrally. This isn’t a purchase order—no prices, no subjective ratings:
- ANCEL AD310—an OBD2 scanner. A corded handheld unit for reading and clearing engine-related codes, including live data and freeze frame.
- Fluke 115—a true-RMS digital multimeter. Measures voltage, resistance, and continuity on sensors, connectors, and wiring.
- BGS 723—an LED test light, 3–48 V. Checks voltage at fuses, connectors, and grounds without loading sensitive control units.
The Starter Just Clicks
A single click when you turn the key points at the power supply in most cases, not at the starter motor. In 80% of cases the cause is the battery or the ground (the ground cable). The order of operations: charge and test the battery first, and only then check the starter motor itself.
The opposite mistake is replacing the starter right away. If voltage sags under load, a brand-new starter will behave exactly the same way.
Battery and IBS Registration
On models from the E60/E90 onward, a battery replacement requires registration in the management system. Without registration, the IBS charges the new battery using the old parameters, the charging profile ends up wrong, and battery life drops sharply.
Registration is part of the battery job and is done right after installation. Don’t put it off: the wrong charging algorithm is active from the very first turn of the ignition.
Symptoms of a Bad Crankshaft Sensor
The classic picture: the engine cranks smoothly and without stumble, but never fires. In the DME, fault 2A99 is logged. That’s the key difference from power-supply problems: there, the starter either doesn’t turn or turns sluggishly—here the mechanics work fine, and the start still doesn’t happen.
Budget Guide
Relative repair cost by component—no specific figures attached:
| Component | Cost Level |
|---|---|
| Battery (including IBS registration) | Low |
| Crankshaft sensor | Low |
| Starter motor | Moderate |
| EWS/CAS repair | Moderate–High |
| High-pressure pump (HDP) | High—comparable to a major repair |
Diagnosis runs from the cheap and likely causes toward the expensive ones, not the other way around.
Frequently Asked Questions
The starter just clicks—is it the battery or the starter?
In 80% of cases the battery or the ground (the ground cable) is at fault. Charge and test first; check the starter only after that.
Does the battery need to be coded after replacement?
Yes, from the E60/E90 onward. Without registration the IBS charges the new battery incorrectly, and its service life drops sharply.
How do you recognize a bad crankshaft sensor?
The engine cranks evenly but won’t start. In the DME—fault 2A99.
The Bottom Line
A BMW that won’t start can almost always be read from one symptom: how the starter behaves and what the fault memory shows. The first move on any no-start is to scan the DME and the CAS and measure battery voltage. Everything else follows the chain—no parts get swapped blind.