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Home / DTC / B2101 — IG SW start POS.circuit low

B2101 — IG SW start POS.circuit low

Detailed page for trouble code B2101.

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Code

B2101

MITSUBISHI B — Body

IG SW start POS.circuit low

Brand: MITSUBISHI
Type: B — Body
AI status
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Page language: EN

Causes

  • Open or short in the ignition switch START position wiring
  • Faulty ignition switch (mechanical/electrical contacts)
  • Poor connector contact or corrosion at the switch or BCM connector
  • Blown fuse or faulty starter/ignition relay controlling the START circuit
  • Poor battery or ground connection causing voltage drop
  • Faulty BCM / control module input circuit

Symptoms

  • Engine does not crank when key is held in START
  • No starter response or intermittent starter engagement
  • Dash accessories light but starter does not activate
  • Starter engages only when key/jiggle applied or after connector manipulation
  • Possible inability to detect START position during diagnostics

What to check

  • Verify battery voltage is healthy (12.4–12.8 V at rest) and battery connections are clean/tight
  • Scan for additional codes and freeze/frame data; clear B2101 and attempt to re‑create
  • Visually inspect ignition switch connector and BCM connector for corrosion, bent pins or loose terminals
  • Check relevant fuses and relays (starter/ignition fuses, starter relay); swap relay with known good where applicable
  • Backprobe the ignition switch START terminal and measure voltage with key in START
  • Measure continuity and resistance between ignition switch START terminal and BCM (with power off)

Signal parameters

  • Ignition switch START terminal: ~0 V with key OFF/ON (not START), ~11–14 V when key held in START
  • Voltage seen at BCM input (START): should match ignition switch output within ~1 V under load
  • Voltage drop >1–2 V between switch and BCM indicates wiring resistance/poor connection
  • Open circuit: infinite/very high resistance between switch START terminal and BCM
  • Short to ground: measured near 0 V at switch output when key in START

Diagnostic algorithm

  1. Confirm the symptom and record all related codes. Attempt to reproduce fault with scan tool connected; note when B2101 sets.
  2. Check battery voltage and main grounds. Repair any low battery/poor ground condition before further testing.
  3. Inspect fuses and starter/ignition relays; replace if faulty. Re-test to see if code returns.
  4. Backprobe ignition switch harness START terminal. With an assistant turn the key to START and measure voltage. Expected ~12 V. If voltage is present at switch output, proceed to step 6.
  5. If voltage is low or absent at the switch output, remove/inspect ignition switch and test switch continuity in START position. Replace ignition switch if internal contacts fail.
  6. If switch output is correct but BCM input is low, disconnect battery and inspect wiring/connectors between switch and BCM for damage, corrosion, pushed-out pins or shorts to ground. Repair wiring as required.
  7. With power off, measure continuity and resistance between the ignition switch START terminal and the BCM START input. Repair any open/high resistance circuits.
  8. If wiring and switch are good, test the BCM input circuit for shorts or failed input drivers. Substitute/replace BCM only after confirming wiring and switch integrity and following manufacturer procedures.
  9. Clear codes and perform a drive/start cycle to confirm the repair. Monitor for recurrence and re-scan for related codes.

Likely causes

  • Intermittent/poor connector at ignition switch harness or start position terminal
  • Wiring shorted to ground between ignition switch and BCM
  • Worn or failed ignition switch contacts (starter circuit)
  • Blown fuse or failed starter/ignition relay
  • Faulty BCM input (less common)

Fault status

⚠️ Status
Ignition switch START position circuit voltage low — START input reads below expected threshold when key is in START. Possible open, short to ground, connector issue, blown fuse/relay, faulty switch, or BCM input fault.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 1.0 - 3.0 hours

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