Engine control module or body control module replaced or reprogrammed without the security password being relearned; Faulty ignition key, transponder or key reader (lock cylinder or ring antenna) so the code is not read correctly; Low battery voltage or a recent battery disconnection that corrupted the stored security data; Wiring or connector fault in the serial data line between the engine control module and the security or body module; Fault inside the body control or security module; Engine control module internal fault or corrupted stored data.

P1503

Theft Deterrent System Password Incorrect.

Cadillac
Powertrain
Manufacturer code
High 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 Cadillac, P1503 means the engine control module received a password or security code from the theft deterrent system that did not match the one it has stored. The engine and security modules failed to authenticate each other, so the engine may be prevented from running as a precaution. It often appears after a module replacement, a battery disconnection or a fault in the security data link or key reading circuit.

Symptoms

Engine cranks but does not start, or starts and stalls after a second or two
Security or theft warning light on or flashing in the instrument cluster
No fuel delivery or no injector pulse while cranking
Intermittent starting problems that come and go
Engine management light on with the code stored

Possible causes

Engine control module or body control module replaced or reprogrammed without the security password being relearnedlikely
Faulty ignition key, transponder or key reader (lock cylinder or ring antenna) so the code is not read correctlylikely
Low battery voltage or a recent battery disconnection that corrupted the stored security datapossible
Wiring or connector fault in the serial data line between the engine control module and the security or body modulepossible
Fault inside the body control or security modulepossible
Engine control module internal fault or corrupted stored datarare

How to diagnose

Read all stored codes and freeze frame data, and note whether other security, body or communication codes are present
Check the battery condition and charging voltage, as low voltage can upset module communication
Observe the security light behaviour when the ignition is switched on and when cranking, and compare it with the expected pattern for that model
Try any spare key to see whether the fault is down to one key or its transponder
Inspect the key reader, its wiring and the connectors at the ignition lock area for damage, corrosion or looseness
Check the data line wiring and connectors between the engine control module and the security or body module, and check module power and earth supplies
Review the vehicle history for recent module replacement, battery work or reprogramming
Use a suitable scan tool to check security data and run the manufacturer's relearn procedure, which varies by model and year

How to fix

Carry out the manufacturer's security or password relearn procedure after module replacement or data loss
Replace or reprogram a faulty key or transponder, or repair or replace the key reader
Repair damaged wiring, connectors or poor earths on the data line and module supplies
Restore a weak battery and make sure the charging system is working properly, then repeat the relearn
Replace the faulty body, security or engine control module and set it up correctly with the vehicle's security system

Can I keep driving?

High severity

The engine may not start or may cut out, so the car is unreliable and may leave you stranded. Do not rely on it for driving until the security fault is diagnosed and the system relearned.

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