B3935 on 2011-2018 Buick Regal: No Start, Security Light, and RF Interference Fixes

What's Unique About the 2011-2018 Buick REGAL

The 2011-2018 Buick Regal, built on the GM Epsilon II platform, shares its sensitive immobilizer system with a wide range of other GM vehicles. This system is notoriously susceptible to Radio Frequency Interference (RFI). General Motors has released multiple, evolving Technical Service Bulletins (TSBs), with PIC5650M being a key version, that specifically address this code across many models. These bulletins strongly advise technicians *not* to replace any parts initially, but instead to investigate for external sources of RFI that could be 'jamming' the signal between the key and the car's immobilizer system.

Diagnostic Flowchart

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

Generation note: This issue affects the fifth-generation Regal (2011-2017 sedan) and the sixth-generation Regal (2018+ Sportback/TourX). The TSBs from GM cover models up to 2019 and beyond, indicating the immobilizer system design and its sensitivity to RFI are similar across these generations and other GM vehicles.

Symptoms You May Notice

• Intermittent no-crank or no-start condition.
• Engine starts and then immediately stalls.
• Security warning light is illuminated or flashing on the dashboard.
• "No Fob Detected" or "Service Theft Deterrent System" message on the driver information center (for vehicles with keyless start).

Most Likely Causes

1. Radio Frequency Interference (RFI) from other devices 🔴 High Probability The vehicle's immobilizer system operates on a low-power radio frequency that is easily overpowered by other electronic devices, a condition documented by GM in TSBs like PIC5650. Common culprits include other vehicle key fobs, building access cards, toll passes, and even aftermarket phone chargers or LED lights.
   How to confirm: Remove all other items from the keychain. Try starting the vehicle with only the Buick key present, physically separated from any other fobs or RFID cards. If the car starts reliably, RFI was the cause.
   Typical fix: Isolate the Buick key from other RFID-enabled cards and key fobs. Avoid using low-quality aftermarket USB chargers or LED lights known to cause interference. This fix is typically free.
   Est. part cost: $0
2. Weak or dead battery in the key fob 🟡 Medium Probability
   How to confirm: Try using the spare key to start the vehicle. If the spare key works reliably, the battery in the primary key is the likely issue. For keyless start models, placing the fob in the designated transmitter pocket may allow it to start even with a weak battery. The pocket location for 2011-2017 models is under the removable cupholder insert in the center console. For 2018+ models, it's a small bin in front of the cupholders. 🎬 Watch: How to start 2018+ models with a dead fob battery.
   Typical fix: Replace the CR2032 battery inside the key fob. 🎬 See this quick walkthrough for replacing your flip key battery.
   Est. part cost: $1-$5
3. Faulty Ignition Key ⚪ Low Probability
   How to confirm: If removing RFI sources and replacing the key battery doesn't work, and the spare key *does* work, the transponder chip in the original key may have failed.
   Typical fix: Replace and reprogram the faulty key. This often requires a dealership or automotive locksmith. OEM Part numbers vary, but common FCC IDs include OHT01060512 for standard ignition and KR55WK50073 for push-to-start models.
   Est. part cost: $85-$250

Rare But Worth Checking

• Faulty Body Control Module (BCM) or BCM Software: While rare, a failing BCM can cause a host of electrical issues, including a no-start condition and setting this code. In some specific cases, particularly when B3935 is found with code B2955, GM has issued a software update for the BCM to correct a no-start condition. This is less common than RFI but is a known issue on some GM models. A faulty BCM is a last resort diagnosis after all other possibilities are exhausted.
• Faulty Immobilizer Antenna/Module: The antenna ring around the ignition cylinder (or the module it connects to) can fail, but this is rare. This would typically cause the issue to be consistent and not intermittent, and it would affect both the primary and spare keys.

Diagnosis Steps

1. Verify the security light is on or flashing when the no-start condition occurs.
2. Remove the Buick key from your keychain. Ensure no other keys, fobs, or RFID cards are near the ignition or center console.
3. Attempt to start the vehicle with the isolated Buick key. If it starts, the issue is RFI.
4. If the vehicle has a keyless start and a 'No Fob Detected' message appears, place the fob in the transmitter pocket and try starting again. For 2011-2017 models, this is under the center console cupholder insert. For 2018+ models, it's in the tray forward of the cupholders.
5. If the issue persists, try the spare key. If the spare key works, the problem is with the original key (likely the battery or a faulty transponder).
6. Replace the battery (typically a CR2032) in the non-working key fob.
7. If neither key works and RFI has been ruled out, a 30-minute security relearn procedure can be attempted. This involves turning the key to 'ON' for 10 minutes until the security light stops flashing, then turning it off and repeating two more times.
🎬 Watch: How to perform the 30-minute security relearn procedure.
8. If the problem continues, the vehicle should be scanned for codes. If B3935 and B2955 are present together, a BCM software update may be required.
9. As a final step, professional diagnosis of the immobilizer module, antenna, and wiring may be necessary.

Parts You'll Likely Need

• Key Fob Battery (OEM #CR2032) — A weak battery is a common cause for intermittent communication between the key and the vehicle's immobilizer.
  Trusted brands: Energizer, Duracell, Panasonic
  OEM price range: $5-$10
  Aftermarket price range: $1-$5
• Replacement Key Fob (OEM #13504205 (Standard Key), 13504283 (Push-to-Start)) — If the internal transponder fails, the entire key fob must be replaced and reprogrammed.
  Trusted brands: ACDelco (OEM), Car Keys Express
  OEM price range: $85-$150
  Aftermarket price range: $40-$90

Related Codes That Often Appear With This One

• B3055 — This is another GM immobilizer code related to the key transponder not being recognized. It is frequently set along with B3935 during the same security event, as noted in TSB PIC5650M.
• B3060 — This code also points to a failure in the transponder system, indicating the security module did not receive a signal from the key. It frequently appears with B3935 and is mentioned in the same TSBs.

Technical Service Bulletins (TSBs) & Recalls

• PIC5650M: Supersedes previous versions and provides an extensive list of potential RFI sources causing no-start conditions and DTC B3935. Advises against parts replacement.
• PIC5650L: An earlier version of the bulletin with similar advice, specifically mentioning not to replace parts and to ask the customer about RFID devices.
• PIC5650B: An older version of the TSB that specifically lists the 2011-2015 Buick Regal and describes the no-start/stall symptoms.
• PI1180A: Addresses a no-start condition with DTCs B3935 and B2955 set, which can be fixed with a BCM software update on some 2014-2015 GM trucks and SUVs. While not for the Regal, it shows a precedent for software fixes for this code.

Platform-Specific Known Issues

• This platform is highly susceptible to Radio Frequency Interference (RFI) from devices like other vehicle key fobs, building access cards, toll passes, and even some aftermarket phone chargers and LED lights.
• GM explicitly advises against replacing parts for this code until all sources of RFI have been eliminated as a potential cause.
• The location of the emergency transmitter pocket for starting with a dead fob battery differs by generation. Pre-2018 models have it under the cupholder insert, while 2018+ models have a marked tray in front of the cupholders.

Mechanic-Grade Diagnostic Values

• Immobilizer Antenna Coil Continuity — expected: A closed circuit (continuity) between the coil's terminals.. Failure: An open circuit (no continuity) indicates a broken coil winding.
• Immobilizer Antenna Signal (Inductive Test) — expected: A specialized tool with an LED light will flash when the key is turned in the ignition.. Failure: If the LED does not flash, it may indicate a problem with the immobilizer coil or the signal from the module. However, a lack of a flash is not a definitive failure on all models.

Scan Tool Commands That Help

• GDS2 (GM Global Diagnostic System 2): Module Diagnostics > Remote Control Door Lock Receiver > Configure/Reset Functions > Add/Replace Key Fobs — This function is used to program new or additional key fobs to the vehicle after ruling out RFI and battery issues.
• GDS2 / SPS (Service Programming System): IMMO Immobilizer Learn - Setup — This procedure is required when replacing the Body Control Module (BCM), Engine Control Module (ECM), or other key immobilizer components. It synchronizes the security information between the modules and the key.
• GDS2 / SPS (Service Programming System): BCM Body Control Module Programming/Configuration — This is used to load the correct software and vehicle-specific options into a new or existing BCM. It must be done before performing the immobilizer learn procedure if the BCM is replaced.

Wiring & Ground Locations

• K89 Immobilizer Control Module — Located on the steering column, as part of the ignition switch assembly.. This is the core component that reads the transponder in the key. A loose connection or failure of this module will directly cause a B3935 code.
• Body Control Module (BCM) — On many similar GM platforms, the BCM is located in the center console, accessible by removing the lower panel on the passenger side footwell.. The BCM is the central computer for the immobilizer system. It receives the signal from the immobilizer module and makes the decision to allow the engine to start. A bad ground or faulty BCM can be a root cause of this code.
• INFO/AIRBAG Fuse — Located in the Right Instrument Panel Fuse Block (X51R).. During a dealer-level immobilizer relearn procedure (SPS IMMO Learn), the OnStar module can interfere and cause the process to fail. GM has a specific procedure that involves temporarily removing this 3-legged fuse and replacing it with a standard 2-legged fuse to isolate the OnStar module while keeping the SIR (airbag) module online for the security handshake.