B3935 on 2019 Buick Regal: Causes and Fixes for Transponder Authentication Error

What's Unique About the 2019-2019 Buick REGAL

The 2019 Buick Regal, built on the GM E2XX platform, uses a sensitive keyless ignition system (Pass-Key III+). The most critical information for this platform comes directly from GM's own Technical Service Bulletins (TSBs). TSBs PIC5650L and PIC5650M explicitly warn technicians *not* to replace parts for this code initially. Instead, they point to a high probability of radio frequency interference (RFI) from common items like other key fobs, access cards, aftermarket LED lighting, or even cheap aftermarket phone chargers, which can "jam" the 125 kHz signal between the key and the car.

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 or no-start condition.
• Engine starts and immediately stalls.
• Security indicator light is illuminated or flashing on the dashboard.
• "No Fob Detected" or similar message on the driver information center.
• Poor or no RKE (Remote Keyless Entry) range.

Most Likely Causes

1. Radio Frequency Interference (RFI) 🔴 High Probability As documented extensively in GM TSBs PIC5650L and PIC5650M, the vehicle's security system is sensitive to external radio signals. These bulletins list numerous sources, including aftermarket LED bulbs, window tint, and even toll passes. The immobilizer system operates at 125 kHz, and RF devices operating in or near this frequency range are the most likely to cause interference.
   How to confirm: Remove all non-essential electronic items from the vehicle and from the key ring. This includes other vehicle key fobs, office access cards, toll passes, and especially cheap, unshielded aftermarket phone chargers plugged into the 12V outlet. Attempt to start the vehicle with only the single Regal key fob present. An RFI meter can also be used by professionals to detect jamming signals.
   Typical fix: Identify and remove the source of the RFI. Avoid keeping multiple RFID-enabled cards or other key fobs on the same ring as the vehicle key. Use high-quality, certified phone chargers. This fix is typically free.
   Est. part cost: $0

Rare But Worth Checking

• Weak or Dead Key Fob Battery: → Shop Vehicle Battery A low battery (typically a CR2032) can cause a weak or inconsistent signal, leading to an authentication failure. This is a simple and inexpensive item to check before pursuing more complex diagnostics.
• Faulty Key Fob: → Shop Keyless Entry Transmitter The internal electronics of the key fob itself can fail from being dropped or water damage. A good way to test this is to try starting the vehicle with a spare key. If the spare key works consistently, the original key is likely the problem.
• Faulty Body Control Module (BCM): → Shop Body Control Module While possible, this is a very rare cause. The TSBs specifically advise against replacing modules unless all other possibilities, especially RFI, have been exhaustively ruled out. On some other GM platforms, a BCM software reflash has been a documented fix for similar issues, but this should be confirmed by a dealer.

Diagnosis Steps

1. Confirm the symptoms: The engine will not crank or starts and stalls, and the security light is on.
2. Isolate the key fob. Remove it from any keychains that have other car keys, office access cards, or RFID-enabled devices.
3. Remove all potential sources of RFI from the vehicle's cabin. Unplug everything from the 12V and USB ports, especially aftermarket phone chargers, dash cams, and power banks.
4. Attempt to start the vehicle with the isolated key fob and all RFI sources removed. If the vehicle starts, you have found the cause.
5. If the issue persists, try the spare key fob. If the spare works, the original key fob's battery is likely dead or the fob itself has failed.
6. Replace the battery in the suspect key fob (typically a CR2032 coin cell 🎬 See this quick walkthrough on replacing your key fob battery.).
7. If the vehicle still won't start with either key and all interference sources removed, place the key fob directly in the transmitter pocket. For the 2019 Regal, this is the small storage bin in front of the cupholders, marked with an icon of a key with radiating lines. 🎬 Watch: How to start your Regal with a dead fob battery. This allows the vehicle to start even with a very weak signal or strong interference.
8. If all the above steps fail, the vehicle may have a hardware issue with the BCM, immobilizer antenna, or wiring, which requires professional diagnosis with a dealer-level scan tool to check for specific symptom bytes or software updates.

Parts You'll Likely Need

• Key Fob Battery (OEM #CR2032) — A weak key fob battery is a possible, though less common, cause for the authentication error. It is an inexpensive and easy replacement to rule out.
  Trusted brands: Energizer, Duracell, Panasonic
  OEM price range: $5-$15
  Aftermarket price range: $2-$10

Related Codes That Often Appear With This One

• B3055 — This is another GM key-related security code ('No Transponder Modulation'), often set alongside B3935 during the same authentication failure event, as noted in TSB PIC5650M.
• B3060 — Also related to the vehicle's immobilizer system ('Unprogrammed Transponder'), this code is frequently mentioned in the same TSBs as B3935.
• C0750, C0755, C0760, C0765, C0775 — These codes are related to the Tire Pressure Monitoring (TPM) system, which also uses RF signals. Their presence alongside B3935 strongly suggests a powerful source of RFI is jamming multiple vehicle systems.
• B3935 57 — This is a more specific version of the code visible on a professional scan tool. The '57' symptom byte explicitly means 'Transponder Authentication Error,' confirming a calculation mismatch after the key was identified, pointing strongly toward a faulty key chip or interference.

Technical Service Bulletins (TSBs) & Recalls

• PIC5650M: Mentions B3935 in the context of an intermittent no crank/no start with the security light on, and lists it with other RFI-sensitive codes like those for the TPMS. It provides a long list of potential RFI sources, including aftermarket LED lights, toll passes, and phone chargers.
• PIC5650L: An earlier version that explicitly warns technicians not to replace parts for this concern and to first investigate if the customer uses any Radio Frequency Identification devices.

Platform-Specific Known Issues

• The primary issue documented by GM for the 2019 Regal and related platforms is the high sensitivity of the Pass-Key III+ or Keyless Access systems to Radio Frequency Interference (RFI). TSBs PIC5650L and PIC5650M specifically instruct technicians to investigate RFI from aftermarket devices before replacing any components.
• A very common real-world cause of RFI on these platforms is a cheap, poorly-shielded aftermarket USB charger plugged into the 12V power outlet. Owners report that simply unplugging the charger permanently resolves the no-start condition.
🎬 Watch: A helpful guide to fixing 'No Fob Detected' errors.

Mechanic-Grade Diagnostic Values

• Immobilizer Antenna Coil Resistance — expected: 7-15 ohms. Failure: A reading outside this range indicates a faulty antenna coil, which is often part of the ignition lock cylinder assembly.
• Immobilizer System Operating Frequency — expected: 125 kHz. Failure: This value is for reference. Interference is most likely from other devices operating in the 120-135 kHz range.

Hidden / Shadow Codes Worth Checking

• B3935 57: Transponder Authentication Error. This is the most common symptom byte. It means the BCM recognized a valid, learned key but the cryptographic challenge/response calculation between the key and the BCM failed. This points strongly to RFI or a faulty transponder chip in the key. (see via GM GDS2 or equivalent professional scan tool.)
• B3935 5A: Invalid Signal. This symptom byte indicates the signal received from the key's transponder was corrupt or not understandable by the BCM. This could point to severe RFI or a failing key fob. (see via GM GDS2 or equivalent professional scan tool.)

Scan Tool Commands That Help

• GDS2: Immobilizer Learn / Key Programming — This function is required after replacing a key fob or the Body Control Module (BCM). It involves a 10-minute security wait time per key and programs the new components to the vehicle's immobilizer system.
• GDS2: Module Diagnostics > BCM > Data Display — To monitor live data from the immobilizer system. A technician can view parameters like 'Key In Ignition Switch Status' or 'Transponder Key Status' to see if the BCM is correctly identifying the key's presence before the authentication attempt fails.
• GDS2: Setup SDM Primary Key in BCM — After replacing the BCM, the new module must learn the primary key from the Sensing and Diagnostic Module (SDM, the airbag module) to clear an airbag warning light (DTC B1001). This is a separate step from the immobilizer relearn.
• Manual Procedure: 30-Minute Security Relearn Procedure — This procedure can sometimes resolve security system glitches without a scan tool. It involves cycling the ignition to 'ON' for three consecutive 10-minute intervals. It is critical to use a battery charger during this process, as any voltage drop can cause it to fail.

Wiring & Ground Locations

• Body Control Module (BCM) — On the E2XX platform (which includes the 2019 Regal), the BCM is typically located in the center console, accessible by removing the passenger side lower trim panel in the footwell. It will have multiple large, color-coded connectors.. The BCM is the central module that manages the immobilizer system. All key authentication signals are processed here. A poor connection at one of its main connectors could potentially cause this issue, though it's less likely than RFI.
• Immobilizer Antenna / Transceiver — The antenna is a coil that is part of the ignition switch assembly or, on push-to-start models, a ring around the start button. Its purpose is to energize and communicate with the key fob's transponder chip.. This antenna sends and receives the 125 kHz signal to the key. Damage to its wiring or a failure of the coil itself (which can be checked by measuring resistance) would prevent key authentication and set a B3935 or related code.
• G201 / G200 — These are the main instrument panel grounds, located at the base of the left (G201) and right (G200) A-pillars, behind the dash structure.. The BCM and other interior modules rely on clean ground connections. While a bad ground is not a primary cause for B3935, intermittent and strange electrical issues accompanying the no-start could point to a ground problem in this area.

Real Owner Repair Stories

• Go-Parts.com repair article for GMC Terrain (2019 GMC Terrain (platform mate)) — Multiple electrical faults, including B3935.
  ❌ Tried (didn't work) Replacing the Body Control Module (BCM)
  ✅ What actually fixed it A GM field engineer diagnosed the issue. Bypassing the Electronic Brake Control Module (EBCM) made the problem disappear. Replacing the EBCM was the final fix, suggesting it was creating massive electrical noise on the CAN bus, interfering with other modules.

OEM Part Supersession History

• 13518368 → N/A — This is a known valid part number for the BCM on a 2019 Buick Regal.
  Heads up: A used BCM from another vehicle will not work without being reprogrammed to the new vehicle's VIN and security credentials. A new BCM must be programmed using GM's SPS software or ordered pre-programmed to the vehicle's VIN.