Code
P1504
ALFA ROMEO
P — Powertrain
Key input fault
AI status
Completed
Completed
100%
Causes
- Dead or weak key battery (remote/transponder)
- Damaged or worn key/transponder (broken chip or casing)
- Faulty or misaligned antenna/coil (ignition barrel/steering column reader)
- Poor wiring, corroded connectors or blown fuse in immobilizer/key circuit
- Faulty Body Control Module (BCM) or immobilizer control unit
- Intermittent or lost CAN/LIN communication between immobilizer and engine/BCM
Symptoms
- Immobilizer or security warning lamp illuminated on dash
- Vehicle refuses to crank or starts then stalls
- Key not detected / ‘Key not recognized’ messages on cluster
- Remote locking/unlocking or keyless entry functions not working
- Intermittent starting or no-crank condition with correct key present
What to check
- Scan vehicle with a capable OEM-level scanner for stored codes and freeze frame data
- Check for other communication U-codes (CAN/LIN) that coincide
- Inspect key: try known-good spare key and check remote/lock functions
- Check key battery voltage (~3.0 V for most transponders/remotes) and replace if low
- Visually inspect antenna/reader (around ignition barrel or push-button reader) and its connector for damage/corrosion
- Verify fuses and supply voltages to BCM/immobilizer module
Signal parameters
- Key/transponder supply voltage: nominal ~3.0 V (internal coin cell)
- Antenna loop resistance: compare to OEM spec (typically a few ohms to tens of ohms)
- Antenna supply (Vbatt) and ground presence at reader connector: 11–14 V when ignition on
- CAN/LIN bus activity: immobilizer and BCM messages present and not reporting errors
- Immobilizer response: valid transponder ID should be read and logged by scan tool (key ID string)
Diagnostic algorithm
- Retrieve P1504 and any additional codes; record freeze frame and time stamps.
- Try spare/programmed key. If spare works, suspect primary key (battery/transponder damage).
- Measure coin-cell battery voltage in suspect key; replace battery and retest. Check remote functions if applicable.
- With OEM scanner, attempt to read key/transponder ID when key presented to reader. Note if reader detects any ID.
- Inspect antenna/reader connector and wiring for corrosion, damage, or loose pins. Repair/clean as needed.
- Measure antenna loop resistance and supply voltages at the reader; compare to OEM specs. Repair wiring or replace antenna if out of spec.
- Check fuses and power/ground circuits for immobilizer/BCM. Repair any electrical faults.
- Monitor CAN/LIN communications for lost messages between immobilizer and BCM/ECM. Repair network faults if present.
- If wiring and reader are good but key ID not accepted, attempt key reprogram/relearn with OEM tool.
- If reprogramming fails or multiple keys not recognized, consider replacing immobilizer module/BCM or antenna per OEM guidance; verify software updates before replacement.
- Clear codes and perform function/test drive to confirm repair.
Likely causes
- Key battery low or key physically damaged
- Antenna loop (reader) fault or connector/wiring problem
- Spare key not programmed or transponder ID not recognized
- Intermittent communication fault on CAN/LIN to immobilizer/BCM
Fault status
Status
Key input fault — transponder/key not detected or not validated by immobilizer/BCM; may prevent engine start or remote functions.
Repair difficulty: Medium
Diagnostic time: 1.0-2.5 hours
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