B3935 on 2020-2025 Buick Encore GX: Security Fault, No-Start Causes and Fixes

What's Unique About the 2020-2025 Buick ENCORE GX

For the Buick Encore GX and related modern GM vehicles on the VSS-F platform, the manufacturer has repeatedly issued Technical Service Bulletins (TSBs) regarding code B3935. The TSBs, including #PIC5650M which directly applies to this vehicle, strongly advise against immediate parts replacement. They emphasize that the immobilizer system is sensitive and that intermittent no-start conditions are frequently caused by external Radio Frequency Interference (RFI) from devices on the owner's keychain or inside the vehicle. The system operates at 125 kHz, a frequency that is easily 'jammed' by other RFID devices and low-quality electronics.

Diagnostic Flowchart

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

Symptoms You May Notice

• Engine will not crank or start
• Engine starts and then immediately stalls
• Security indicator light is illuminated or flashing on the dashboard
• "No Key Detected" message on the driver information center
• "Service Theft Deterrent System" message on the driver information center.

Most Likely Causes

1. Radio Frequency Interference (RFI) 🔴 High Probability As noted in GM TSBs (PIC5650 series), the immobilizer system is highly sensitive to external radio signals. Common sources include other key fobs, building access cards, toll passes, aftermarket phone chargers, USB power banks, and even some aftermarket LED lights plugged into the car.
   How to confirm: Remove the Encore GX key from the keychain so it is completely isolated. Also, unplug any aftermarket devices from the 12V or USB ports. Attempt to start the vehicle with only the single key present. If it starts, RFI was the cause.
   Typical fix: Isolate the vehicle key from all other electronic devices. This fix is free.
   Est. part cost: $0
2. Weak or Dead Key Fob Battery 🟡 Medium Probability A weak battery may not have enough power to complete the cryptographic 'handshake' with the vehicle, especially in the presence of minor RFI.
   How to confirm: Try the spare key fob. If the spare key works reliably, the battery in the primary key is the likely culprit. You can also try placing the weak fob in the transmitter pocket to start the car. For the Encore GX, this is a small depression in the tray on the center console, just in front of the main storage area. Place the fob in this spot with the buttons facing up and try to start the engine. 🎬 Watch: How to start your Encore with a dead fob battery.
   Typical fix: Replace the CR2032 battery inside the key fob. 🎬 See this quick walkthrough for replacing your key fob battery.
   Est. part cost: $2-$10
3. Faulty Key Fob ⚪ Low Probability The internal transponder chip or circuitry can be damaged from being dropped or from water exposure.
   How to confirm: If the spare key works perfectly and the primary key fails even after a battery change and isolation from RFI, the primary key is faulty.
   Typical fix: Replace the key fob. This requires purchasing a new key and having it professionally programmed to the vehicle by a dealer or automotive locksmith.
   Est. part cost: $150-$400

Rare But Worth Checking

• Faulty Immobilizer Antenna: The antenna ring around the ignition switch (or in the center console for push-button start) can fail, preventing it from energizing the key's transponder. Diagnosis involves checking the antenna's resistance (typically 7-15 ohms).
• Body Control Module (BCM) Failure: In rare cases, the BCM itself can fail. This is usually a last resort after all other possibilities, including wiring and ground issues, have been exhausted. This would likely be accompanied by other seemingly unrelated electrical problems.
• Wiring or Ground Issues: A poor ground connection for the BCM can cause a wide range of erratic electrical behaviors, including communication errors like B3935.
• BCM Software Glitch: On some other GM platforms, a similar issue was caused by a software glitch requiring a dealer to re-flash the BCM. While less commonly documented for the Encore GX specifically, it remains a possibility if all other causes are ruled out.

Diagnosis Steps

1. Isolate the Key: Remove the Encore GX key from your keychain, away from all other keys, fobs, and access cards. Try to start the vehicle. This is the most important first step.
2. Check for RFI Sources: Unplug all aftermarket devices from USB and 12V ports (phone chargers, dash cams, LED lights, etc.) and try starting again.
3. Try the Spare Key: Attempt to start the vehicle with your second/spare key to rule out a problem with the primary key fob.
4. Use the Transmitter Pocket: If the key fob battery is suspected to be weak, place the fob in the dedicated transmitter pocket. In the Encore GX, this is the small tray/depression on the center console in front of the storage compartment. Attempt to start the engine normally.
5. Replace Key Fob Battery: If the spare key works, or if the primary key is old, replace the CR2032 battery.
6. Scan for All Codes: Use a professional-grade scanner to check for codes in all modules, especially the BCM. Note any other codes like B3055, B3060, or TPMS codes.
7. Inspect BCM Wiring and Grounds: If the problem persists, a technician should inspect the wiring and ground connections for the Body Control Module for any signs of corrosion or damage.
8. Follow TSB #PIC5650M: A professional technician should consult this bulletin for the official GM diagnostic procedure for this specific intermittent no-start condition, which includes using tools to detect RFI.

Parts You'll Likely Need

• Key Fob Battery (CR2032) — A weak battery is a common cause for the key's transponder failing the security handshake.
  Trusted brands: Energizer, Duracell, Panasonic
  OEM price range: $5-$10
  Aftermarket price range: $2-$5
• Key Fob / Transmitter (OEM #13532753 (FCC ID: HYQ4EA, 433 MHz) or 13530513 (FCC ID: HYQ4ES, 434 MHz). Part number should be verified by VIN.) — If the key's internal transponder chip is damaged, the entire key fob must be replaced and programmed.
  Trusted brands: ACDelco, GM Genuine Parts
  OEM price range: $150-$300
  Aftermarket price range: $50-$150

Related Codes That Often Appear With This One

• B3055 — This code means 'No Transponder Modulation or No Transponder,' indicating the system didn't detect a key at all. It often appears with B3935 as part of the same security event, especially due to RFI.
• B3060 — This code means 'Unprogrammed Transponder,' suggesting the key is not learned by the vehicle. It's also frequently seen with B3935 in GM service bulletins.
• C0750, C0755, C0760, C0765, C0775 — These are Tire Pressure Monitor (TPM) sensor codes. TSB PIC5650M lists these alongside B3935 because the TPM system also uses RF signals (315/433 MHz) and can be affected by the same RFI sources that disrupt the 125 kHz immobilizer signal.

Technical Service Bulletins (TSBs) & Recalls

• PIC5650M: Addresses intermittent no crank, no start, or start/stall concerns with the security light on, specifically mentioning DTC B3935 and advising investigation for Radio Frequency Interference (RFI) before parts replacement.

Platform-Specific Known Issues

• As documented in TSB #PIC5650M, the 2020-2025 Buick Encore GX is susceptible to intermittent no-start or start-and-stall conditions with the security light on, often caused by RFI and logging code B3935.
• The designated backup location to start the car with a dead key fob is a small tray on the center console, directly in front of the main storage area, which is unique to the GX and Envista models.

Mechanic-Grade Diagnostic Values

• Immobilizer Exciter Coil Primary Resistance — expected: 0.3 - 1.0 ohms. Failure: A reading outside this range indicates a faulty coil.
• Immobilizer Exciter Coil Secondary Resistance — expected: 8,000 - 11,500 ohms (8-11.5 kOhms). Failure: A reading outside this range, particularly an open circuit (infinite resistance), points to a bad coil.
• Generic GM Immobilizer Antenna DC Resistance — expected: ~6 ohms. Failure: Significant deviation from this value suggests a problem with the antenna winding.

Hidden / Shadow Codes Worth Checking

• B3935 57: This symptom byte specifies 'Transponder Authentication Error'. It confirms the BCM recognized a learned key, but the cryptographic challenge/response calculation failed. This points strongly toward a faulty key chip or severe RFI, rather than an unprogrammed key. (see via GM GDS2 or other professional scan tool capable of reading manufacturer-specific symptom bytes.)
• B3935 3A: This symptom byte means 'Incorrect Transponder Key'. This indicates the key's transponder ID itself was not found on the BCM's list of learned keys, which would happen if using a key from another vehicle. (see via GM GDS2 or other professional scan tool.)

Scan Tool Commands That Help

• GDS2 (GM Global Diagnostic System 2): Immobilizer Learn / Key Programming — This function is required after replacing a key fob or the Body Control Module (BCM). It involves a 10-12 minute security wait time and guides the technician through adding new keys to the vehicle's memory. An online connection to GM's SPS (Service Programming System) is typically required.
• GDS2 (GM Global Diagnostic System 2): Module Diagnostics > BCM > Output Controls — While not specific to B3935, bidirectional controls allow a technician to command various BCM outputs (lights, locks, etc.) to verify the BCM itself is functioning correctly, helping to rule out a complete module failure.
• GDS2 (GM Global Diagnostic System 2): Add/Replace Key Fobs (under RCDLR module) — This function within the Remote Control Door Lock Receiver module is used to program the remote (RKE) functions of the key fob, which is a separate procedure from programming the immobilizer transponder chip.

Wiring & Ground Locations

• Body Control Module (BCM) — In the center console, accessible from the front passenger side footwell after removing the lower trim panel.. The BCM is the central module that manages the immobilizer system. All key authentication signals are processed here. A loose connector, corrosion, or internal BCM fault can cause B3935.
• G103 — Located at the left rear of the engine compartment on the cowl, generally above the brake booster.. This is a primary ground point for the BCM, Instrument Panel Cluster (IPC), and Data Link Connector (DLC). A loose or corroded connection at G103 can cause intermittent communication errors and various electrical issues, including security codes like B3935.
• Keyless Entry Antennas — Multiple small antennas are located around the vehicle, including in the center console, bumpers, and door panels, as part of the Passive Entry Passive Start (PEPS) system.. While a failure of these antennas more commonly causes a 'No Remote Detected' message, a fault in the center console antenna could potentially interfere with or prevent the BCM from authenticating the key, contributing to a B3935 code.

Real Owner Repair Stories

• Camaro5 Forum user 'RedTexasSS' (2012 Chevrolet Camaro SS (Uses a similar GM Pass-Key III+ system)) — No crank, no start, 'Service Theft Deterrent System' message, security light on.
  ❌ Tried (didn't work) Disconnecting and reconnecting the battery., An initial attempt at the 30-minute key relearn procedure.
  ✅ What actually fixed it A professional scan revealed code B3935. The original switchblade-style key had failed internally. Replacing the key with a new one and having it programmed to the vehicle resolved the issue completely.

Model Year Variations Within This Range

• 2024-2025: The 2024 model year introduced a significant interior refresh, replacing the previous infotainment system with a large, integrated 19-inch digital dashboard (11-inch touchscreen and 8-inch driver display). While the underlying immobilizer logic remains the same, this new system runs on GM's Global B electrical architecture, which is more software-intensive. Some early 2024 models experienced software glitches causing the screens to go black, which were addressed by a software update/recall. This highlights the increased importance of checking for software updates when diagnosing electrical issues on newer models.