BMW Injector Short-to-B+ Faults Explained: How to Diagnose and Fix Like a Pro

BMW Injector Short-to-B+ Diagnosis & Repair Guide | Electronic Repair Egypt

If you’re a BMW technician or DIY enthusiast, encountering DTCs like 112404 (Cylinder 4 Short to B+) and 112104 (Cylinder 1 Short to B+) can be frustrating. These injector faults may seem complicated, but with the right approach, you can safely pinpoint the problem and restore your BMW to perfect running condition.
Here’s a detailed, step-by-step guide from an expert perspective, focusing on both technical accuracy and practical workflow.

​BMW injectors are controlled by the DME (Digital Motor Electronics) through low-side switching. A “Short to B+” fault indicates that the injector pin sees battery voltage (12V) constantly, instead of the pulsed signal expected from the DME.

Common DTCs associated with this issue:

• 112404 – Injector Cylinder 4, activation: Short to B+
• 112104 – Injector Cylinder 1, activation: Short to B+

Without the right diagnostics, technicians might replace parts unnecessarily. But the fault is usually electrical: either injector coil shorted, wiring damaged, or DME driver MOSFET blown.

Step 1: Safety FirstBefore touching injectors or the DME:

• Disconnect the battery negative terminal.
• Work on a cold engine.
• Use a multimeter for safe measurement.

Safety prevents further damage and protects the DME electronics.

Step 2: Inspect Wiring and ConnectorsStart with the obvious: visual inspection.

• Check harness insulation for chafing, melted sections, or pin corrosion.
• Focus on cylinders 1 and 4.
• Damaged wiring is the most common culprit for Short-to-B+ faults.

Pro Tip: Even minor wire contact with 12V supply can trigger these DTCs.

Step 3: Test Injector Coils

• Disconnect the injectors from the DME.
• Measure resistance across each injector:
  • Typical petrol injector: 12–16 Ω
  • 0 Ω → shorted coil → replace the injector
  • Open circuit → injector broken internally

This step immediately identifies whether the fault lies inside the injector itself.

Step 4: Check Wiring from DME to Injectors

• With injectors disconnected:
  • Test for continuity from DME output pin to +12V → should be open.
  • Continuity indicates wiring short to battery → trace and repair.
  • Also check for shorts to ground.

Correct wiring ensures the DME driver is not being overloaded by external shorts.

Step 5: Diagnose DME Output DriverIf injectors and wiring are confirmed good:

• Measure DME output pins using a resistive dummy load (never direct 12V).
• Normal operation: pulsed signal to the low-side switch.
• If output is stuck at 12V → DME driver MOSFET blown → DME repair or replacement required.

💡 Note: The BDC does not directly control injectors, so if all wiring and injectors are good, the DME is the main suspect.

Step 6: Clear DTCs and Test
After repairing or replacing faulty components:

• Clear DTCs using ISTA/D .
• Test engine start and idle.
• Confirm injectors fire correctly and DTCs do not return.

Advanced Resources for BMW Technicians
For professionals aiming to master BMW ECU diagnostics, we highly recommend Electronic Repair Egypt’s advanced ECU hardware and repair courses. These courses cover everything from injector driver repair to complete DME and BDC diagnostics.
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