BMW E90/E91/E92/E93 ECU/DME

BMW E90/E91/E92/E93 ECU/DME Post-Repair Troubleshooting Guide

Important: Use this guide if you reinstalled your repaired BMW E90/E91/E92/E93 ECU/DME and the problem is still happening.
Important: Please do not open/disassemble the module. Opening the module can damage it and may void warranty coverage.

Common concerns this guide applies to:

• No start / crank-no-start / intermittent start
• Communication faults with the DME
• Rough running, power loss, limp mode
• Throttle/engine warning lights
• Internal DME fault codes (examples: 29CC, 29CD, 29CE, 29CF, 29D0)

Step 1: Confirm correct module and install basics

• Confirm the DME part number matches the original vehicle application.
• Confirm the DME connectors are fully seated and locked (lever fully closed).
• Inspect for bent pins, pushed-back pins, moisture, corrosion, or oil intrusion at the DME connectors.
• If the DME was removed from an enclosure/e-box, confirm all seals/covers are installed correctly to prevent water intrusion.

Step 2: Battery health and voltage stability

• Verify battery voltage is healthy before cranking.
• If the vehicle cranks slowly, clicks, or voltage drops heavily during crank, address battery/terminal/cable issues first.
• Low voltage can trigger multiple DME faults and communication issues.

Step 3: Verify fuses and main power feeds related to DME
Check all fuses related to:

• DME/ECU/Engine electronics
• Ignition (Terminal 15)
• Main relay / DME relay
• Fuel pump relay/fuse (no-start symptoms)
  If a fuse is blown:
• Do not keep replacing it—inspect for shorts in wiring, coils, injectors, sensors, or water intrusion in the e-box.

Step 4: Ground and power connection quick inspection

• Inspect the main engine ground strap(s) (engine to chassis).
• Check the battery ground connection and chassis ground points for looseness/corrosion.
• A weak ground can cause communication errors, no-start, and random fault sets.

Step 5: Scan the vehicle and record faults (before clearing)

• Perform a full vehicle scan (Engine + CAS/immobilizer + transmission + ABS).
• Save all codes and freeze-frame data.
• Clear codes once, then re-scan after a short start attempt / short drive.
• Focus on the faults that return immediately.

Step 6: If the issue is “No communication with DME”

• Confirm the vehicle’s diagnostic port has power and the scan tool connects to other modules.
• If other modules communicate but DME does not:
  • Re-check DME power/ground fuses/relays.
  • Re-check DME connector seating and pin condition.
  • Inspect the e-box area for water intrusion (common cause of DME power/communication faults).
• If no modules communicate, suspect vehicle-side power/ground or CAN/K-line network issues, not the DME.

Step 7: If the vehicle cranks but won’t start

• Verify fuel delivery basics (fuel pump operation and fuel pressure if possible).
• Verify ignition basics (coil connectors seated, coils not unplugged, no obvious harness damage).
• Check for unplugged sensors commonly disturbed during service:
  • MAF sensor
  • Crankshaft/camshaft sensors
  • Throttle body connector
• If the issue began immediately after other work, re-check what was touched first.

Step 8: If you have rough idle, hesitation, or power loss

• Inspect intake tube/boots for cracks and loose clamps (unmetered air causes drivability faults).
• Check vacuum hoses/PCV connections.
• Confirm MAF connector is locked and wiring is intact.
• If throttle body or battery was disconnected, the vehicle may need adaptation/relearn procedures (handled by a capable scan tool).

Step 9: If the same “internal fault” codes return (29CC–29D0 examples)

• Clear codes and drive/start again to confirm they truly return under the same conditions.
• If they return immediately with stable voltage and no power/ground issues found:
  • Treat this as a strong sign of a remaining vehicle-side trigger (voltage drop, water intrusion, harness/connector issue) OR a persisting internal DME fault.
• Prioritize checking voltage stability, e-box moisture, and connector pin fitment first (most common causes of repeat internal faults).

Step 10: Intermittent problems (most common reason for repeat symptoms)

• With the engine idling (if it runs), gently wiggle the harness near the DME and e-box area.
• If symptoms change, suspect harness damage, poor pin tension, corrosion, or water intrusion.
• Inspect the harness for chafing where it passes through brackets/edges.

Step 11: What to send UpFix if the issue continues

• VIN
• Exact symptoms (when it happens, hot/cold, crank/no-crank, intermittent/constant)
• Full fault code list from a complete scan (and which codes return immediately after clearing)
• Any recent work performed (battery replacement, alternator, starter, valve cover/oil leaks, e-box opened, wiring repairs)
• Photos of DME connector pins and any signs of moisture/corrosion in the e-box area

If these steps don’t resolve the concern, the next best step is hands-on diagnosis of vehicle power/ground, e-box moisture, and harness integrity, since those are the most common causes of repeat DME-related symptoms after reinstall.