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Home / DTC / B29EA — Key switch signal:open cycle

B29EA — Key switch signal:open cycle

Detailed page for trouble code B29EA.

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Code

B29EA

MITSUBISHI B — Body

Key switch signal:open cycle

Brand: MITSUBISHI
Type: B — Body
AI status
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Completed 100%
Page language: EN

Causes

  • Open or intermittent wiring (broken wire, chafing, connector fault) between key switch and control module
  • Corroded or loose connector at the ignition/key switch
  • Faulty ignition/key switch (mechanical or electrical failure)
  • Poor battery or low system voltage during crank
  • Faulty body/control module (BCM/ECU) input circuit
  • Aftermarket alarm/immobiliser or starter interrupt device interfering with the signal

Symptoms

  • Engine may not crank or start intermittently
  • No accessory power from key-on positions at times
  • Security or immobilizer indicator may illuminate
  • Intermittent loss of electrical functions tied to key positions (ACC, IGN)
  • DTC present in memory, possibly only after repeated attempts

What to check

  • Read and record DTC and freeze frame data with a capable scan tool; clear codes and attempt to reproduce
  • Visually inspect ignition/key switch, connectors, and wiring for corrosion, damage, or loose pins
  • Check related fuses and ground points for tightness and corrosion
  • Measure battery voltage at rest and during crank to rule out low-voltage artifacts
  • Probe the key switch signal at the switch connector and at the module connector for continuity and expected voltages
  • Wiggle test wiring/connectors while monitoring signal to reproduce intermittent behaviour

Signal parameters

  • Expected: key switch signal transitions between ~0 V (open/inactive) and battery voltage (~12–14 V) when closed
  • During crank: stable high/active voltage near battery voltage (allowing for voltage drop under load, typically >9 V)
  • Fault condition: prolonged open (0 V) when a closed/high signal is expected or unexpected interruptions during a cycle
  • Module detection: open-cycle duration threshold varies by model; intermittent opens of tens to hundreds of milliseconds can trigger the code

Diagnostic algorithm

  1. Connect scan tool, note B29EA and any other codes. Clear codes and attempt to reproduce with a known-good battery.
  2. Inspect ignition switch assembly and harness for visible damage, corrosion, or aftermarket devices in-line with the circuit.
  3. Verify battery voltage and charging system health to eliminate low-voltage causes.
  4. With ignition OFF, disconnect connector at the ignition/key switch. Check for continuity between switch pins and the module harness where applicable.
  5. With the switch connected, backprobe the key switch signal pin. Verify proper voltages for OFF/ACC/ON/CRANK positions and that the signal does not drop out unexpectedly. Use a scope if available to see transient opens.
  6. Perform a wiggle/stress test on harness and connector while monitoring the signal to catch intermittent opens.
  7. If wiring and connectors are good but the signal is still incorrect at the module, inspect and test the module input circuit or consult wiring diagrams for continuity to the module. Check for signs of water intrusion or corrosion at the module connector.
  8. If an aftermarket immobilizer or starter interrupt is present, temporarily disable or isolate it and re-test.
  9. Repair damaged wiring or connector faults. Replace the ignition/key switch if bench-tested faulty. Replace or repair control module only after wiring and switch are verified good.
  10. After repairs, clear codes and verify the fault does not return through multiple key cycles and road test as required.

Likely causes

  • Damaged wiring or connector to the key/ignition switch (most likely)
  • Worn or failed key/ignition switch
  • Intermittent connector corrosion or poor pin contact
  • Aftermarket immobiliser wiring interrupting the signal
  • Less likely: internal module input failure

Fault status

⚠️ Status
B29EA — Key switch signal: open cycle. The control module detected the key/ignition switch signal opening when it should be closed (or dropping out during a cycle). This points to an open/intermittent circuit, switch fault, or interference from aftermarket equipment. Further testing required to locate the cause.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 0.5-2.0 hours

Similar codes

B29E0 B29E1 B29E2 B2900 B2910
406

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