Key transponder not read correctly (worn, damaged or wrong key, or weak signal at the ignition barrel); ECM replaced or swapped from another car and not yet set up to match the vehicle's security system; Fault in the theft deterrent or body control module, or its communication with the ECM; Wiring or connector fault on the serial data line between the security module and the ECM, or poor earths; Low battery voltage or a recent power interruption corrupting the security data exchange; Internal ECM fault stopping it from storing or comparing the password.

P1503

Theft Deterrent System Password Incorrect.

Saturn
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 Saturn, P1503 means the engine control module received an incorrect password from the vehicle's theft deterrent system. The ECM and the anti-theft module must exchange a matching security code before the engine is allowed to keep running. If the code is missing, corrupted or does not match, the ECM sets this fault and may limit or cut fuel and starting.

Symptoms

Engine cranks but does not start
Engine starts then stalls after a second or two
Security or theft warning light on or flashing in the instrument cluster
Intermittent no-start that comes and goes
Fault appears after a battery change, key work or module replacement

Possible causes

Key transponder not read correctly (worn, damaged or wrong key, or weak signal at the ignition barrel)likely
ECM replaced or swapped from another car and not yet set up to match the vehicle's security systemlikely
Fault in the theft deterrent or body control module, or its communication with the ECMpossible
Wiring or connector fault on the serial data line between the security module and the ECM, or poor earthspossible
Low battery voltage or a recent power interruption corrupting the security data exchangepossible
Internal ECM fault stopping it from storing or comparing the passwordrare

How to diagnose

Read all stored codes and freeze frame data, and note any other communication or security-related codes
Check battery condition and charging voltage, as low voltage can upset module communication
Try every available key, inspect them for damage, and check the security light behaviour when the ignition is turned on
Inspect the ignition barrel, transponder reader and their wiring and connectors for damage, corrosion or looseness
Check the serial data line and the ECM and security module connectors, power feeds and earths
Find out whether the ECM or any related module has recently been replaced or reprogrammed
Check the security system status and data using a scan tool capable of reading the security module, if the model supports it
Refer to model-specific procedures, as the security relearn process varies by model and year

How to fix

Replace a worn or faulty key, or have a correctly coded key supplied and programmed
Perform the manufacturer's security relearn or password synchronisation procedure for the model after module replacement or power loss
Repair damaged wiring, connectors or earths in the security and data circuits
Replace a faulty transponder reader or ignition barrel assembly
Replace the security or body control module if it is confirmed faulty, then set it up to match the vehicle
Replace and correctly program the ECM only if testing confirms it is faulty

Can I keep driving?

Medium severity

If the car starts and runs normally, it can be driven, but get it diagnosed soon because it may fail to restart. If it stalls or will not start, it should not be relied upon until the security fault is fixed.

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