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Home / DTC / B1A01 — KOS key2 internal error

B1A01 — KOS key2 internal error

Detailed page for trouble code B1A01.

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B1A01

MITSUBISHI B — Body

KOS key2 internal error

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

Causes

  • Weak or dead key fob battery (KEY2)
  • Damaged or defective key transponder (KEY2)
  • Faulty KOS antenna/read coil or wiring to the antenna
  • Poor power or ground to the KOS control module
  • Corroded/loose connectors at KOS module, antenna or key reader
  • Software fault or corrupted data in KOS/BCM/immobilizer module

Symptoms

  • Ignition push-button won’t start or engine not recognized when using KEY2
  • Security/immobilizer warning lamp illuminated or flashing
  • Intermittent recognition of KEY2 (works sometimes, fails other times)
  • No response from push-button start for that key, but other keys may work
  • Remote functions (lock/unlock) may still work while KOS start not recognized

What to check

  • Read and record all stored DTCs and freeze frame data with a capable scanner
  • Try starting with the other registered key(s) to confirm if problem is limited to KEY2
  • Check key fob battery voltage (replace if below nominal ~3 V for coin cell types)
  • Inspect antenna/read coil location, connectors and harness for damage or corrosion
  • Verify vehicle battery voltage and KOS module supply and ground pins with ignition on
  • Check CAN/ LIN communication errors and verify KOS module is seen by the network

Signal parameters

  • Key fob coin cell battery voltage (target ≈ 3.0 V for usual CR2032-type fobs)
  • Vehicle battery voltage with ignition on (should be ≈ 12–14.5 V)
  • KOS module supply and ground continuity and voltage at connector pins
  • RF/transponder read pulses at the antenna coil when key is presented (presence/absence or amplitude change)
  • CAN bus messages between KOS and ECM/BCM — presence of periodic ID and immobilizer status frames
  • Module self-diagnostics or status bytes reported by factory scan tool

Diagnostic algorithm

  1. Connect a factory-level or capable OBD scanner; record DTCs and freeze frame data. Note whether code is stored as current or history.
  2. Verify whether the fault affects only KEY2 by testing with a known-good spare key. If spare key works, focus on the KEY2 fob.
  3. Replace the KEY2 coin cell battery and retest recognition. Inspect key internal PCB and antenna coil for damage or corrosion.
  4. Inspect antenna/read coil and its wiring/harness around the start area for physical damage or poor connections. Wiggle-test connectors while observing live data or scanner status.
  5. With ignition off, check power and ground at the KOS module connector. With ignition ON, verify proper supply voltage and CAN communication lines for faults (use multimeter/scan tool).
  6. Use oscilloscope or scan-tool live-data to verify transponder read pulses at the antenna when KEY2 is presented; compare with KEY1 or a working key if possible.
  7. If wiring, power/ground and antenna are good, attempt KOS module reset/clear codes and re-learn procedure per factory service manual. Reattempt key registration and start test.
  8. If code persists after keys and wiring verified, check for software updates or TSBs; perform module reprogram/reflash if recommended by manufacturer.
  9. If module still fails and diagnostics point to internal fault, replace KOS/immobilizer module or antenna unit as per service manual. After replacement, perform key registration and final verification.
  10. Clear DTCs and perform repeated start tests and road test to confirm repair. Re-scan to ensure no related network codes remain.

Likely causes

  • Key fob battery depleted (most common)
  • Damaged transponder or key electronics
  • Faulty/shifted antenna coil or poor connector
  • Module power/ground supply problem
  • KOS module internal fault or corrupted programming

Fault status

⚠️ Status
KOS (Keyless Operation System) reports KEY2 internal error — KEY2 not recognized or internal module/read error detected. Check key battery, antenna, wiring, and KOS module.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 1.0-3.0 hours

Similar codes

B1A00 B1A02 B1A03 U0100 U0073
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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