GM Security Codes and the Passlock Relearn

Which relearn family your car uses

GM used four anti-theft families. The family decides whether a no-tool relearn even applies.

1. Passlock I and II, no chip in the key, 1995 to 2007 trucks, SUVs, vans and most cars. The 30 minute relearn works.
2. Passkey III, also called PK3, black-head transponder key, 1999 to 2007. The 30 minute relearn works.
3. Passkey I, also called VATS, resistor pellet in the key blade, 1986 to 1996 F-body, Corvette and Caprice. Resistance based, so there is nothing to learn. A worn pellet or dirty lock cylinder contacts is a key or cylinder job.
4. Immobilizer, transponder or push-button start, 2008 and newer, and every 2014 and newer. No 30 minute relearn. These need a scan tool and programmed keys.

Pull the code first and confirm the family before you spend half an hour on a procedure that does not apply.

What the security system does before the engine runs

GM theft deterrent systems check that the key belongs to the car before they let the engine run. On the older systems the lock cylinder sends a resistance value to the body control module (BCM). On the newer ones a transponder in the key head sends a code the BCM reads through an exciter coil. Once the BCM trusts the key, it swaps an encrypted password with the powertrain control module (PCM). Only after that exchange does the PCM enable fuel injector pulse, and on some cars the starter circuit. If the password never arrives, the engine may crank and not fire, or it may not crank at all. The code you see is the PCM reporting a failed handshake, not naming a part to replace.

The security codes this catalog carries

These are the theft deterrent codes on this site, grouped by what they tell you.

• P1626 and P1629: the PCM never received the fuel or start enable password. These are the most common no-start security faults.
• P1630: the BCM is still in learn mode. Usually a relearn that was never finished.
• P1631: the start enable signal arrived but was wrong. A worn Passlock sensor or a bad password is the usual cause.
• P1632: the BCM is actively cutting fuel after a failed authentication.
• B3031: the security controller is stuck in learn mode.
• B2721: the PASS-Key detection circuit, the resistor pellet side of the older system.
• B3055: no transponder modulation, so the key itself was not read.

The generic side of an immobilizer no-start is P0513. Read both layers on the scan tool.

The 10 minute and 30 minute cycles

The full procedure is three 10 minute cycles. That is where the 30 minute name comes from.

Before you begin

Before you begin, put a battery charger or maintainer on the car. The ignition sits ON for the whole wait with no alternator output, and a marginal battery will drop voltage and fail the learn. Have every key you want to use in hand, because the relearn erases all previously learned keys. Turn off or unplug aftermarket remote start and alarm modules so a bypass does not fire mid-procedure.

Relearn steps

1. Key to ON without starting the engine. The security light comes on.
2. Wait 10 minutes, or until the security light goes out. The cycle ends when the light goes out, even if that is early.
3. Key fully to OFF for 5 seconds, not ACC.
4. Repeat the ON wait and the OFF pause twice more.
5. Turn the key off. The new code is written on the next start cycle. Start the engine.

When the fault is mechanical, not the relearn

A relearn only teaches a good system a new code. If the hardware cannot produce a stable signal, the relearn will not hold. The parts that wear are the Passlock sensor in the lock cylinder housing, the exciter coil around the cylinder, the key pellet or transponder, and the column harness that flexes with the tilt wheel. An aftermarket remote start or alarm spliced into the security wiring is a common cause of intermittent lockouts.

Two signs point at hardware. If the security light never goes out even past 15 minutes, check the Passlock sensor and its wiring before the BCM. If the car relearns and the light comes back a day later, you have an intermittent sensor or a chafed harness. The relearn masked it, it did not fix it. B2721 and B3055 point at the key and exciter side.

What a DIYer can do, and what needs a scan tool

Safe at home: load test the battery, run the relearn with a maintainer, try a known-good spare key, take other transponder keys off the ring, and inspect the column connector for a chafed or spliced harness. These cover most older Passlock and PK3 no-starts.

Needs a scan tool, a locksmith or a dealer: any 2008 and newer immobilizer, every 2014 and newer car, cutting and programming a new key, and replacing a BCM or PCM. Those modules must be programmed to the car, and a used module will not carry the right security data. Do not cut the security wires or bypass the system. A bypass can leave the car unable to start at all and hard to sort out later.

Stop and call for help when the security light will not follow the procedure, the car locks out again right after a completed relearn, or you are replacing a module.

Mistakes that trigger another lockout

• A weak battery. The wait is long, voltage sags, and the learn aborts. Charge or replace first.
• Not taking the key fully to OFF between cycles, or holding it at ACC.
• An aftermarket remote start or alarm firing during the wait.
• Using an unprogrammed key and expecting a relearn to bless it. The key must already be the right type for the car.
• Relearning one key and forgetting the rest. The procedure wipes the old keys, so every key gets relearned or stops working.
• Treating the relearn as a fix for an intermittent sensor. The fault returns. P1630 often means someone stopped a relearn early.