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Home / DTC / B1A0B — KOS key4 performance

B1A0B — KOS key4 performance

Detailed page for trouble code B1A0B.

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B1A0B

MITSUBISHI B — Body

KOS key4 performance

Brand: MITSUBISHI
Type: B — Body
Views: UK: 32 EN: 31 RU: 34
AI status
Completed
ready
Completed 100%
Page language: EN

Causes

  • Weak or dead battery in the key fob/transponder
  • Damaged or worn transponder chip inside the key
  • Poor antenna/coil reception in the ignition/key reader (antenna ring)
  • Loose, corroded, or disconnected wiring/connectors between KOS antenna and control module
  • Faulty KOS/immobilizer control module or BCM
  • Software/configuration or pairing issue between key and module

Symptoms

  • Vehicle fails to recognize Key 4 or intermittent recognition
  • Engine cranks but does not start when Key 4 is used, or no-crank condition depending on vehicle logic
  • Immobilizer/KEY warning lamp illuminated or flashing
  • Push-button start not responding to that specific key
  • Other keys may work normally while Key 4 shows problems
  • Intermittent starting or security-related shutdowns

What to check

  • Retrieve freeze-frame and full DTC list with a factory-level scan tool and note any related codes
  • Try starting/using vehicle with a different registered key to confirm if issue is limited to Key 4
  • Check key fob battery voltage and replace if below specification
  • Visually inspect Key 4 for physical damage or water ingress
  • Inspect antenna ring/reader around the ignition or start button area for damage, loose screws, or disconnection
  • Check wiring harness and connectors between antenna and immobilizer/KOS module for corrosion, bent pins, or looseness

Signal parameters

  • Key fob battery voltage (V)
  • Antenna coil/reader supply voltage (V) and ground continuity
  • Transponder response waveform (RF/transponder signal) — amplitude and timing
  • CAN/FlexRay messages between immobilizer module and ECU (presence and error frames)
  • Immobilizer/KOS module supply voltage and ground
  • Entry/recognition event logs from immobilizer (if available on scan tool)

Diagnostic algorithm

  1. Confirm the DTC (B1A0B) with a capable diagnostic tool and record related codes and freeze-frame data.
  2. Verify symptom with another known-good registered key. If other keys work normally, focus on Key 4 (skip to step 5).
  3. Check and replace the battery in Key 4. Retest recognition and attempt start.
  4. Visually inspect Key 4 for physical damage or signs of moisture. If damaged, try a spare or replacement key.
  5. Inspect antenna/reader assembly at the ignition switch or push-button area: check for loose connectors, damaged coil, or corrosion. Wiggle-test connectors while monitoring recognition.
  6. Measure antenna/reader supply and ground voltages at the immobilizer connector and at the antenna. Verify within specification. Repair wiring/grounds as needed.
  7. Use an oscilloscope or manufacturer diagnostic tool to observe the transponder response waveform when Key 4 is presented. Compare to a known-good key waveform.
  8. Check immobilizer/KOS module communication on the vehicle network (CAN) and resolve any bus errors. Repair wiring or module as required.
  9. Attempt key re-learn/programming for Key 4 following manufacturer procedure. If re-learn fails repeatedly, suspect key/transponder or module fault.
  10. If antenna and wiring are confirmed good and reprogramming fails, consider replacing the key (transponder) then retest. If problem persists, consider replacing immobilizer/KOS module or consult dealer technical support for module replacement or software update.
  11. Clear codes and verify repair by repeating start/recognition tests multiple times and under different conditions (engine hot/cold).

Likely causes

  • Low key battery (most common)
  • Faulty transponder chip in key 4
  • Antenna coil around ignition switch or reader has poor connection or is damaged
  • Connector corrosion at immobilizer/KOS harness
  • Key lost its learned data (needs reprogramming)
  • Intermittent wiring fault to immobilizer module

Fault status

⚠️ Status
KOS Key4 performance fault — key 4 transponder/recognition problem detected. Check key battery, antenna/reader, wiring, and immobilizer module.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 0.5-3.0 hours

Similar codes

B1A0A B1A0C B2797 B2799 U0121
406

Browse 406 MITSUBISHI manuals: repair procedures, diagnostics, wiring diagrams, component locations, service data and Labor Times by year, model and trim.

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