Ignition key transponder not read correctly or faulty key; Poor connection or fault in the data line between the ECM and the immobiliser or body control module; Replacement or second-hand ECM or body module not learned to the vehicle's security system; Faulty immobiliser antenna or ignition lock cylinder reader; Low battery voltage or poor earths causing communication errors during start; Internal fault in the ECM or immobiliser module.

P1503

Theft Deterrent System Password Incorrect.

GMC
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

P1503 on a GMC means the theft deterrent system password is incorrect. The engine control module (ECM or PCM) and the body or immobiliser module did not exchange a matching security password or code during the start sequence. The ECM therefore treats the immobiliser as not authorised and may limit or stop engine running.

Symptoms

Engine cranks but does not start
Engine starts then stalls after a second or two
Security or theft deterrent warning light on or flashing
No crank in some cases, depending on model
Intermittent no-start that comes and goes
Check engine light on

Possible causes

Ignition key transponder not read correctly or faulty keylikely
Poor connection or fault in the data line between the ECM and the immobiliser or body control modulelikely
Replacement or second-hand ECM or body module not learned to the vehicle's security systemlikely
Faulty immobiliser antenna or ignition lock cylinder readerpossible
Low battery voltage or poor earths causing communication errors during startpossible
Internal fault in the ECM or immobiliser modulerare

How to diagnose

Read all stored and pending codes and freeze frame data, and note any communication or security codes in other modules
Check the battery state and charge level, and check the main power and earth connections, as low voltage can upset module communication
Try a second known good key and note whether the security light behaviour changes
Inspect the ignition lock area, antenna ring and its connector for damage, moisture or looseness
Inspect the ECM and body module connectors and the security data wiring for corrosion, bent pins or damage
Use a scan tool to view security or immobiliser status and data, and check that the modules are communicating
Find out whether the ECM, body module or keys have been replaced recently, as they may need to be learned to each other
Check the manufacturer's procedure for your model, as the security relearn method varies by year and system

How to fix

Replace or reprogramme a faulty or unlearned key so the transponder is recognised
Repair damaged wiring, connectors or earths in the security data circuit
Replace a faulty antenna or ignition lock reader as needed
Carry out the manufacturer's security or password relearn procedure after fitting a replacement ECM or body module
Charge or replace a weak battery, and repair poor power or earth connections
Replace the ECM or immobiliser module only once other causes are ruled out, and have it programmed to the vehicle

Can I keep driving?

Medium severity

If the car starts and runs normally, it can be driven, but it may fail to restart, so get it checked soon. If it will not start, it cannot be driven until the security fault is fixed.

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