GM Immobilizer Relearn
You installed a programmed BCM or ECM, every accessory works, and the truck cranks but won't start with Theft Deterrent System on the dash. You tried the 30-minute relearn three times and nothing changed. This page explains why, and what actually fixes it.
Why the 30-minute relearn does nothing here
The 10-minutes-times-three procedure is a Passlock relearn. Passlock reads a resistance value from a sensor in the steering column, and the module can learn that value on its own. 2014 and newer GM vehicles don't use Passlock. They use a transponder immobilizer: the key or fob carries an encrypted ID, and the BCM has to be taught that ID and then handshake with the ECM using a shared secret. Neither of those can be created by cycling the ignition.
The security light going out at the 11-minute mark is not success. That's the BCM's tamper timer expiring. It will do exactly the same thing on the next cycle, and the next, forever. Stop running it — you're just draining the battery.
What we can and can't do on the bench
When we program a module, SPS2 writes the calibrations, your VIN, and your option configuration. That's why everything that was broken or missing before now works correctly. What SPS2 does not write is the immobilizer secret and the transponder key table — GM only allows that to be established with the module in the vehicle, live on the bus, with a real key present. There is no bench path to it on this architecture, from us or from anyone else.
So the module is doing the right thing. It's announcing that it has no key data and is waiting to be taught.
The codes you'll see
| Code | What it means |
|---|---|
| B2955 | Security system sensor data incorrect — the BCM isn't getting valid transponder data |
| B3031 | Security controller in learn mode — the BCM is waiting to learn a key and never finishes |
| B3055 | Key not present — no transponder read at all |
| B3935 | Transponder authentication error |
| B3902 | Incorrect immobilizer identifier received — often sets in the EBCM, SDM or cluster too |
Any of these after a module install points at the same missing step, not at a bad module. A good sanity check: if the vehicle starts on the old module, your key, exciter coil and column wiring are all fine, and the problem is entirely the security data in the new one.
Four ways to get it done
1. Mobile automotive locksmith — usually the easiest
They come to the vehicle, which matters if you're dealing with an RV, a box truck, or anything that won't fit in a dealer bay. Ask for a GM immobilizer learn after BCM replacement, all keys present. Tools that handle it: Autel IM608 Pro / IM508S with XP400, Advanced Diagnostics Smart Pro, AutoProPad G3. Give them your VIN and have them confirm coverage before they dispatch — medium duty and chassis cab trucks are a common gap in those coverage lists. Typically $150–250.
2. GM dealer or a shop with Techline Connect and an MDI 2
The factory path. Ask the service writer for the post-module-replacement Immobilizer Learn — in SPS it's listed under Z1 Immobilizer Learn – Setup, then Body Control Module IMMO Learn with Existing Transponder or Remote Key. Roughly half an hour of labor plus the mandatory wait. For medium duty, call a truck or fleet dealer rather than a light-duty store.
3. Buy the access yourself
An ACDelco TDS subscription — a two-day pass is enough — plus a J2534 interface gets you Techline Connect and the same SPS procedure. Worth it if you're a shop that will do this again. Not worth it for one vehicle; signup and activation can take a day on its own.
4. Send us both modules
If your original module still powers up and the vehicle starts on it, ship it to us with the replacement. We move the security data across at the memory level and keep the configuration work already done. You reinstall and it starts — no scan tool ever touches the vehicle. This is the only option that requires nothing on your end but shipping.
Before whoever runs it shows up
- Every key and fob for the vehicle on hand. If the tech has to run Key Learn (New), it deletes all keys first and re-learns only what's present.
- A battery maintainer on the vehicle. The procedure holds ignition on through a hard 12–13 minute security timer that cannot be shortened or skipped.
- Aftermarket remote start, alarm, and bypass modules disabled or unplugged.
- Tell them to pick the existing transponder / existing key option if the keys are your originals. Key Learn (New) is for lost or replacement keys and is more work than you need.
2019+ Global B is different again
On Global B vehicles — 2019+ T1XX trucks and most of the current lineup — a junkyard BCM is not a starting point at all. SPS2 won't move the immobilizer or keyless entry data off the donor VIN, and these modules aren't readable on the bench because the security handshake requires the real vehicle bus. Those need either a virgin module from a dealer, set up in the vehicle, or your original module cloned into the used one before it goes back in.
Not sure which category your vehicle falls into?