B3935 on 2007-2014 GMC Acadia: Security System Faults, Causes, and Fixes

What's Unique About the 2007-2014 Gmc ACADIA

The first-generation Acadia, along with its Lambda platform-mates (Buick Enclave, Chevy Traverse, Saturn Outlook), is notably sensitive to Radio Frequency Interference (RFI). Manufacturer documents, specifically TSB PIC5650, warn that devices like other car keys, keyless access fobs, building access cards, or even some aftermarket phone chargers on the same keychain can interfere with the signal between the ignition key and the vehicle's immobilizer antenna. This often leads to an intermittent no-start condition that can be difficult to replicate once the source of the interference is removed, causing confusion for both owners and technicians.

Diagnostic Flowchart

Tap your situation to follow the diagnostic path that matches what you're seeing on this vehicle.

Symptoms You May Notice

• Intermittent no-crank, no-start condition.
• Engine starts and immediately stalls.
• Security indicator light (a car with a lock symbol) 🎬 Watch: See what a security light no-crank looks like on an Acadia. is illuminated or flashing on the dashboard.
• Message on the driver information center like 'Service Theft Deterrent System'.
• Key is not recognized, remote functions may also fail.

Most Likely Causes

1. Radio Frequency Interference (RFI) 🔴 High Probability The vehicle's Passkey III+ immobilizer system operates at 125 kHz, a frequency easily disrupted by other RFID devices. GM TSB PIC5650C specifically lists toll passes, gate/door access cards, fuel station speed passes, and even other vehicle keys as potential culprits.
   How to confirm: Remove all other keys, fobs, and access cards from the keychain. Try starting the vehicle with only the single Acadia key. If the vehicle starts 🎬 Watch: This quick fix shows how removing RFI sources solves the issue. consistently, RFI was the cause. This is the first step any technician should take.
   Typical fix: Keep other RFID-emitting devices separate from the ignition key. No parts replacement is necessary.
   Est. part cost: $0
2. Defective or Damaged Ignition Key 🟡 Medium Probability → Shop Ignition Switch The transponder chip embedded in the key head can fail or be damaged from being dropped, preventing it from responding correctly to the TDM's security challenge. The key housing can also wear out, causing a poor connection.
   How to confirm: Attempt to start the vehicle with a known-good spare key. If the spare key works reliably, the primary key is faulty. If both keys fail, the issue is likely with the vehicle itself.
   Typical fix: Replace the faulty key. A new key must be cut and programmed to the vehicle by a dealer or an automotive locksmith with the proper equipment.
   Est. part cost: $75-$200
3. Faulty Body Control Module (BCM) ⚪ Low Probability → Shop Body Control Module Software glitches or internal failures in the BCM can prevent it from correctly reading the immobilizer information or communicating with other modules. Water intrusion into the cabin, particularly near the passenger-side footwell where the BCM is often located, can also cause failure.
   How to confirm: This requires a professional diagnosis with a scan tool to check for communication errors with the BCM. A specific TSB (PI1180A) points to a BCM software update for this code on some GM models, though it primarily lists newer trucks/SUVs. The principle remains valid for the Acadia.
   Typical fix: Reprogram the BCM with updated software using GM's SPS. In some cases, the BCM itself may need to be replaced and programmed.
   Est. part cost: $400-$800

Rare But Worth Checking

• Faulty Theft Deterrent Module (TDM) or Antenna: → Shop Anti-Theft Control Module The TDM is the module that reads the key. If it fails, or if the antenna ring around the ignition cylinder is damaged, it cannot communicate with the key. This is typically diagnosed after ruling out the key and RFI.
• Wiring Harness Issues: Damaged or corroded wiring between the TDM, BCM, and ECM can disrupt the security signal, though this is less common than other causes. This can be caused by previous repairs or rodent damage.
• Low Battery Voltage: → Shop Vehicle Battery A weak or failing battery can cause a multitude of electrical issues, including communication errors between security modules during startup. If the battery voltage drops too low during cranking, the TDM may not authenticate the key properly.

Diagnosis Steps

1. Check for the security light on the dashboard. If it's on or flashing, the issue is with the anti-theft system.
2. Remove all other keys, fobs, and RFID cards from the GMC Acadia key ring as per TSB PIC5650M. This includes toll passes, work ID badges, and other car keys.
3. Try starting the vehicle with just the single key. If it starts, the problem is RFI.
4. If the problem persists, try using a spare key. If the spare key works, the original key is defective.
5. If neither key works, attempt the 30-minute relearn procedure 🎬 Watch: Follow this step-by-step guide for the GM security relearn procedure. described in the 'DIY-Friendly Fixes' section.
6. If the relearn procedure fails, check the vehicle's battery to ensure it is fully charged and healthy. Low voltage can cause module communication errors.
7. If all else fails, the vehicle needs professional diagnosis. A technician will use a scan tool to read codes from the Theft Deterrent Module (TDM) and Body Control Module (BCM).
8. The technician will check for available software updates for the BCM, as noted in some GM service bulletins.
9. If no software update is available, further diagnostics will test the TDM, the ignition cylinder's antenna ring, and associated wiring.

Parts You'll Likely Need

• Transponder Ignition Key (OEM #15824471 (Supersedes 89024363, 15824470)) — The transponder chip inside the key is often the point of failure after RFI is ruled out. The key uses a 'Circle Plus' PK3+ chip.
  Trusted brands: ACDelco, Strattec (5903089)
  OEM price range: $100-$200
  Aftermarket price range: $50-$120
• Body Control Module (BCM) (OEM #20935349 (Part number can vary by year and options, check VIN)) — In some cases, a software glitch or internal failure requires the BCM to be reprogrammed or replaced.
  Trusted brands: ACDelco (GM Genuine), Dorman
  OEM price range: $400-$600
  Aftermarket price range: $250-$450

Related Codes That Often Appear With This One

• B3055 — Relates to a problem with the key transponder. Often set alongside B3935 when there is a security communication issue.
• B3060 — Indicates an issue with the transponder antenna circuit. It frequently appears with B3935 when there's an RFI problem or a hardware fault in the key reading system.
• B2955 — This code, related to the security sensor data circuit, is mentioned in TSB PI1180A as appearing with B3935 when a BCM software update is needed on certain GM vehicles.

Technical Service Bulletins (TSBs) & Recalls

• PIC5650M: Mentions B3935 as part of a group of DTCs related to intermittent no-crank/no-start and security light issues.
• PIC5650L: Advises technicians not to replace parts for B3935 until checking for Radio Frequency Identification devices on the customer's keychain.
• PI1180A: Notes that a BCM software update can resolve a no-start condition where B3935 and B2955 are set together on some GM models (primarily newer trucks/SUVs, but the principle is relevant).
• PIC5650C: A later version of the RFI bulletin which provides an extensive list of potential interference sources, including aftermarket LED lights and phone chargers.

Platform-Specific Known Issues

• Technical Service Bulletin PIC5650M mentions B3935 in relation to an intermittent no-crank/no-start with the security light on.
• Technical Service Bulletin PIC5650L explicitly warns against replacing parts for this code until Radio Frequency Interference has been ruled out as the cause.
• Owner forums for the Acadia and its platform mates are filled with threads about this issue. A common story involves a user whose car won't start, they get it towed to a shop where it starts perfectly, only for the problem to return. This is a classic sign of intermittent RFI from an object on the keychain that was removed before the technician saw the vehicle.

Mechanic-Grade Diagnostic Values

• Immobilizer Exciter Coil Function — expected: Using an inductive tester (transponder coil tester), an LED on the tool should flash when placed near the ignition lock cylinder and the key is turned to ON.. Failure: If the LED on the tester does not flash, it indicates the Theft Deterrent Module is not energizing the antenna coil, pointing to a problem with the TDM or its wiring/power/ground.

Hidden / Shadow Codes Worth Checking

• B3935 57: The '57' is a symptom or status byte indicating 'Transponder Authentication Error'. It specifies that the key's transponder was read successfully, but the cryptographic challenge-and-response calculation failed. This confirms the issue is not a simple 'key not read' problem (like B3055 or B3060) but a specific authentication failure. (see via A professional GM scan tool like a Tech2 or GDS2.)

Scan Tool Commands That Help

• Tech2 / GDS2: Service Programming System (SPS) — This function is used to update the software/calibration of the Body Control Module (BCM). It is the required fix for cases where B3935 is accompanied by B2955, as per TSB PI1180A on related GM platforms.
• Tech2 / GDS2: Immobilizer Learn > Add/Replace Keys — This function is required after replacing a key or the Body Control Module (BCM). It programs the unique transponder values of the keys into the vehicle's memory. This is a more robust method than the 30-minute manual relearn and is necessary when replacing modules.

Wiring & Ground Locations

• Body Control Module (BCM) — Under the driver's side of the dashboard, to the left of the steering column.. The BCM is the central hub for the Passkey III+ system. It processes the key data from the Theft Deterrent Module (TDM) and communicates with the ECM to enable or disable starting.
• G103 — On the cowl at the left rear of the engine compartment, typically above the brake booster.. This is a primary ground point for the Body Control Module (BCM), Instrument Panel Cluster (IPC), and Data Link Connector (DLC). A poor connection here can interrupt power to the BCM or corrupt data communication, leading to false theft codes.
• G218 — On the front of the dash panel, inside the vehicle. It may be necessary to pull back insulation to access it.. A TSB for similar GM platforms (PIT5405C) identifies a poor ground at G218 as a cause for a wide range of electrical issues, including no-start conditions and BCM communication DTCs. The dash insulator mat can get trapped under the ground eyelet, causing high resistance.
• Theft Deterrent Module (TDM) / Antenna — The TDM is the immobilizer reader itself, which contains the exciter coil (antenna). It is located around the ignition lock cylinder, behind the steering column shrouds.. This component energizes the key transponder and reads its signal. All communication from the key flows through this module and its wiring harness to the BCM.

Real Owner Repair Stories

• YouTube channel 'Dallas Auto Diag' (2013 GMC Acadia) — No-crank, no-start with the security light on. Multiple other modules were also offline, including HVAC and radio.
  ❌ Tried (didn't work) Initial diagnosis focused on the theft system modules.
  ✅ What actually fixed it The low-speed GMLAN serial data wire was being shorted to voltage by a faulty internal component in the HVAC control module. Because the BCM and TDM communicate on this same data bus, the short prevented them from talking to each other, triggering the security system. Disconnecting the HVAC control module restored communication and allowed the vehicle to start. The final fix was replacing the HVAC control module.
• Reddit user in r/GMCACADIA (2010-2012 GMC Acadia) — Car went into anti-theft mode (no crank/no start) after being used to jump-start another vehicle.
  ❌ Tried (didn't work) Replacing the ECU (Engine Control Unit) did not solve the problem.
  ✅ What actually fixed it The 30-minute key relearn procedure successfully resolved the issue. The procedure involves turning the key to 'ON' for 10 minutes (until the security light goes out), then 'OFF' for 5 seconds, repeating this cycle a total of three times. This re-synchronized the existing key to the vehicle's security system.

OEM Part Supersession History

• 20935349 → 84922494 — Standard part lifecycle update; may contain hardware or firmware revisions.
  Heads up: When replacing a BCM, the new module must be programmed to the vehicle using the Service Programming System (SPS) and the keys must be relearned. Failure to program will result in a no-start condition.
• 22756010 → Unknown — This is the part number for the separate Theft Deterrent Module (immobilizer antenna/reader) on some models.
  Heads up: If this module is replaced, a key relearn procedure is required. On some models, a full BCM setup procedure with a scan tool may be necessary.

Model Year Variations Within This Range

• 2013-2014: The Acadia received a significant facelift for the 2013 model year, which included updates to the interior, dashboard, and underlying electronics. This can lead to differences in module locations, wiring, and software compared to 2007-2012 models. The documented case of an HVAC module shorting the GMLAN bus occurred on a 2013 model.
• 2014+: While the TSB (PI1180A) for a BCM software update to fix B3935 and B2955 was for newer GM trucks/SUVs starting in 2014, it indicates that GM was aware of and fixing software-related authentication issues around this time. It is plausible that later 2013-2014 Acadias may have similar software logic or may have an unlisted update available.