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P1503

Theft Deterrent System Password Incorrect.

  • Chevrolet
  • Powertrain
  • Manufacturer code
  • Medium severity

How this code is built

P
SystemPowertrain
1
Defined byThe manufacturer
5
AreaVehicle speed, idle control and auxiliary inputs
03
FaultThe specific fault in that area

What it means

On a Chevrolet, P1503 means the engine control module has received an incorrect password from the theft deterrent (immobiliser) system. The module and the body or security control unit did not agree on the security code during the start-up handshake, so the engine may be prevented from running. The exact modules involved and the way the password is exchanged depend on the model and year.

Symptoms

  • Engine cranks but will not start, or starts and stalls after a second or two
  • Security or theft warning light on or flashing in the instrument cluster
  • No-start that comes and goes, especially after the battery has been disconnected or run flat
  • Engine control module not communicating properly with other modules
  • Fault returns soon after being cleared

Possible causes

  • Engine control module replaced or swapped from another vehicle and not yet learned to the car's security systemlikely
  • Security or immobiliser relearn not completed after battery disconnection, low battery voltage or a repair to the systemlikely
  • Faulty or poorly connected ignition key transponder, or a key not programmed to the vehiclepossible
  • Wiring or connector fault in the data line between the engine control module and the body or security control unitpossible
  • Fault in the immobiliser or body control module, or its reader coil around the ignition lockpossible
  • Internal fault or corrupted memory in the engine control modulerare

How to diagnose

  1. Read all stored codes and freeze frame data from every module, noting any security, communication or low voltage codes alongside P1503
  2. Check the battery condition and charging voltage, as low voltage can interrupt the security handshake
  3. Ask about recent work: battery disconnection, module replacement, key changes or ignition lock repairs
  4. Try each programmed key in turn and note whether the security light behaves differently with any of them
  5. Inspect the connectors and wiring to the engine control module, body control module and ignition lock reader for damage, corrosion or loose pins
  6. Use a scan tool to view live security system data and confirm whether the modules report a password or learn status mismatch
  7. Check the data line between modules for continuity and shorts, following the wiring diagram for the specific model
  8. Confirm the correct part number and security status of any replacement module

How to fix

  • Carry out the manufacturer's security relearn or password synchronisation procedure for that model, using suitable diagnostic equipment
  • Programme or replace faulty or unrecognised keys so they match the vehicle
  • Repair damaged wiring, connectors or poor earths in the security data circuit
  • Replace a faulty ignition lock reader coil or body control module, then complete the required security programming
  • Replace or have the engine control module properly reprogrammed if it is faulty or not matched to the vehicle, then perform the security learn procedure
  • Recharge or replace a weak battery before repeating the relearn

Can I keep driving?

Medium severity

If the car starts and runs normally, it can be driven to a workshop, but it may fail to restart at any time, so book it in soon. If it will not start or stalls straight away, it cannot be driven until the security fault is resolved.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.

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