GM Passlock Anti-Theft Security Module

(Chevy Express / GMC Savana 1998–2002, Silverado / Sierra 3500 2001–2002 – P/N 16264975)

If you still have issues after reinstalling your repaired module, work through this checklist before assuming the module is bad again.

1. Confirm the basics

• Vehicle is within the supported years/models.

• Module part number on the label is 16264975 (or same family).

• Connectors are fully seated and locking tabs engaged.

• No bent, pushed-out, or corroded pins at the module plugs.

2. Battery, grounds, and fuses

• Battery key-off voltage: 12.4–12.6 V. Charge if low.

• Check all related fuses in under-hood and interior boxes (IGN, CRANK/START, ECM/PCM, SECURITY/BCM, ECM B, etc.).

• Verify solid grounds at the module: less than 0.1 V drop between module ground and battery negative while cranking.

3. Scan tool: read codes first

Use a scan tool that can talk to PCM/ECM and Security/Passlock/VTD.

Write down:

• Every code

• Which module set it

• Whether it is current or history

Common situations:

• P1626 Theft Deterrent Fuel Enable Signal Not Received

  • PCM is not seeing a valid fuel-enable signal.

• U-codes or “No Communication” with security module

  • Scan tool can’t talk to the module.

• No theft-related codes but still a no-start

  • Likely a non-theft issue (fuel, ignition, other wiring).

Clear codes only after writing them down.

4. Passlock / VTD relearn (30-minute procedure)

A relearn is often required if the PCM, security module, ignition switch/Passlock sensor was replaced, or power/codes were recently cleared.

1. Try to start the engine, then release key to ON (engine will stall or not start).

2. Leave key ON for about 10 minutes until the security light stops flashing or goes out.

3. Turn key OFF for at least 5 seconds.

4. Repeat steps 1–3 two more times (total of 3 key-on cycles).

5. After the third cycle, turn key OFF once more, then start the engine.

• If the engine starts and keeps running, the relearn is complete.

• If it still stalls or sets P1626, continue with the checks below.

5. Ignition switch & Passlock sensor

These vans/trucks frequently have issues at the ignition lock cylinder / Passlock sensor, not just the module.

• Inspect the wiring at the ignition lock cylinder (Passlock sensor wires).

• Look for:

  • Broken, stretched, or chafed wires

  • Loose or partially unplugged connector

  • Symptoms that change when you wiggle the column harness or key

If moving the harness makes the security light or starting behavior change, the sensor or its wiring is suspect and may need repair or replacement. Repeat the relearn after any repair here.

6. Wiring between security module and PCM

If communications to the security module look good but P1626 or similar issues persist:

• Using a wiring diagram, check continuity on the fuel-enable / VTD signal and any data lines between the module and PCM.

• Look for:

  • High resistance or open circuits

  • Corrosion or moisture under the dash at connectors

  • Aftermarket alarm or remote-start splices in the same circuits

Repair any wiring faults, clear codes, and rerun the relearn.

7. When to contact UpFix

The module may need to be re-evaluated if:

• You cannot communicate with the security module even though power and grounds are confirmed good.

• The module blows a fuse, gets unusually hot, or shows new physical damage.

When contacting support or posting on the forum, please include:

• Order number and VIN

• Year / make / model / engine

• Full list of DTCs (which module set them, current vs history)

• Confirmation that the 30-minute relearn was completed and how it behaved

• Voltage/ground readings at the module during key-on and cranking

• Any ignition-switch, Passlock sensor, or wiring repairs already performed

This information helps us quickly determine whether the issue is still vehicle-side or if your module needs warranty testing.