GMC Sierra Denali B3935 Code (2017-2020): No Start, Security Light Causes & Fixes

What's Unique About the 2017-2020 Gmc SIERRA DENALI

The 2017-2020 Sierra Denali, spanning the K2XX and T1XX platforms, uses a GM immobilizer system known for its sensitivity to Radio Frequency Interference (RFI) at its 125 kHz operating frequency. GM has issued multiple Technical Service Bulletins (TSBs), including the PIC5650 series, explicitly warning technicians *not* to replace expensive modules like the BCM as a first step. The problem is so frequently caused by external electronic signals from other keys, toll passes, aftermarket LED lights, or phone chargers that the primary diagnostic step is to question the driver and check for these sources of interference.

Diagnostic Flowchart

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

Generation note: The 2017-2020 range includes the end of the third generation (K2XX; 2017-2018 models) and the start of the fourth generation (T1XX; 2019-2020 models). While the underlying electrical architecture changed with the T1XX platform, the immobilizer logic and its high sensitivity to RFI that causes code B3935 are common issues affecting both generations, as confirmed by the applicability of TSBs like PIC5650M to both platforms. [⭐ MANUFACTURER TSB — highest authority] Bulletin #PIC5650M

Symptoms You May Notice

• Intermittent no-crank, no-start condition. [⭐ MANUFACTURER TSB — highest authority] Bulletin #PIC5650M
• Engine starts and then immediately stalls. [⭐ MANUFACTURER TSB — highest authority] Bulletin #PIC5650M
• Security indicator light is illuminated on the instrument cluster. 🎬 See this guide on resetting the Sierra theft deterrent system. [⭐ MANUFACTURER TSB — highest authority] Bulletin #PIC5650M
• "No Fob Detected" message on the Driver Information Center (DIC).

Most Likely Causes

1. Radio Frequency Interference (RFI) 🔴 High Probability GM has documented this platform's sensitivity to RFI in multiple TSBs (PIC5650 series). The immobilizer system operates in a frequency range that can be disrupted by other RFID devices like building access cards, other vehicle fobs, toll passes, and even aftermarket electronics like phone chargers and LED lights.
   How to confirm: Remove all other keys, key fobs, access cards, and toll passes from the keychain. Unplug any aftermarket devices (phone chargers, dash cams, etc.) from the 12V outlets. Attempt to start the vehicle with only the Sierra key present. If it starts, RFI was the cause.
   Typical fix: Isolate the ignition key from other electronic devices. In one documented case for a similar GM vehicle, an aftermarket USB charger was the culprit. No parts are needed.
   Est. part cost: $0
2. Weak or Defective Key Fob 🟡 Medium Probability The transponder chip in the key can be damaged from being dropped, or the internal battery can weaken, causing an intermittent signal. A weak signal is more susceptible to failure during the security challenge, especially in the presence of minor RFI.
   How to confirm: Try starting the vehicle with a known-good spare key. If the spare key works consistently, the primary key is faulty. You can also try replacing the battery in the suspect key fob (typically a CR2032) as a first step.
   Typical fix: Replace the faulty key fob and have it professionally programmed to the vehicle. If the battery is the issue, simply replace the battery.
   Est. part cost: $5-$15 for a battery, $150-$400 for a new programmed key fob.
3. Body Control Module (BCM) Software Glitch ⚪ Low Probability For earlier models in this generation range (specifically 2014-2015), GM issued TSB PI1180A for a similar issue where B3935 and B2955 are set due to a software logic error. While the TSB doesn't explicitly cover 2017-2020, a software glitch remains a possibility, especially if both codes are present.
   How to confirm: This can only be confirmed by a GM dealer or a qualified shop with access to GM's Service Programming System (SPS). They can check the current BCM calibration against the latest available version on TIS2WEB.
   Typical fix: Reprogram the BCM with the latest software calibration. This is a software fix, not a hardware replacement.
   Est. part cost: $0 (labor cost only)

Rare But Worth Checking

• Faulty Immobilizer Antenna (Exciter Coil): The antenna ring around the ignition cylinder (or in the center console for push-to-start models) energizes the key's transponder. While uncommon, damage to this coil or its wiring can prevent the key from being read correctly, leading to B3935.
• Poor BCM Ground Connection: The Body Control Module relies on clean ground connections to function. A loose or corroded ground can cause a host of unpredictable electrical issues, including security system faults. On K2XX trucks (2017-2018), ground G218 located behind the driver's side kick panel is a known weak point that can cause various electrical issues. A Reddit user with a 2019 T1XX truck fixed a no-crank issue by replacing a disintegrated engine-to-frame ground strap under the passenger side.

Diagnosis Steps

1. Isolate the Key: Remove the Sierra key from your keychain. Ensure no other keys, fobs, or RFID cards are nearby. This is the most crucial first step. [3, 4, ⭐ MANUFACTURER TSB — highest authority] Bulletin #PIC5650J
2. Remove RFI Sources: Unplug all aftermarket devices from 12V power outlets, especially USB chargers and dash cams, as these are known to cause interference. [6, 18, ⭐ MANUFACTURER TSB — highest authority] Bulletin #PIC5650M
3. Try the Spare Key: Attempt to start the vehicle with your second key. 🎬 Watch: Troubleshooting a no-start and performing a key relearn. If it works reliably, the first key is the problem.
4. Replace Key Fob Battery: If you don't have a spare, replace the battery in the key fob (typically CR2032) as a low-cost diagnostic step.
5. Check Vehicle Battery: Ensure the main vehicle battery is fully charged (above 12.4 volts) and that the terminals are clean and tight. Low system voltage can cause various electronic module errors.
6. Inspect Grounds: For persistent issues, inspect known problem ground locations. On 2017-2018 models, check G218 behind the driver's kick panel. On all models, check the main engine-to-frame and battery-to-chassis grounds for corrosion or damage.
7. Scan for Codes: Use a professional-grade scanner to confirm B3935 and check for other related codes like B3055, B3060, or B2955.
8. Seek Professional Diagnosis: If the above steps fail, the vehicle needs to be diagnosed by a technician who can view live data from the BCM and test the immobilizer system components directly.

Parts You'll Likely Need

• Key Fob Battery (OEM #CR2032) — A weak battery can cause an intermittent signal, leading to authentication failure. It's the cheapest and easiest potential parts-based fix.
  Trusted brands: Energizer, Duracell, Panasonic
  OEM price range: $5-$15
  Aftermarket price range: $2-$10
• Keyless Entry Remote / Key Fob (OEM #ACDelco 13580082 (for K2XX, 2017-2018) or 84312372 (for T1XX, 2019-2020). Verify by VIN.) — If the internal transponder chip in the key has failed, the entire key fob must be replaced and programmed to the vehicle. Part numbers vary by year and features.
  Trusted brands: ACDelco, Standard Motor Products
  OEM price range: $150-$250
  Aftermarket price range: $70-$150

Related Codes That Often Appear With This One

• B3055 — This code means 'No Transponder Key/Key Not Learned' and is often set with B3935 because the system may first fail to read the key (B3055) before failing the authentication challenge (B3935). It is mentioned in TSB #PIC5650M. [1, 3, ⭐ MANUFACTURER TSB — highest authority] Bulletin #PIC5650M
• B3060 — This code points to an issue with the transponder antenna circuit or an unprogrammed key. It can appear with B3935 if the signal from the key is weak or corrupted by interference. It is also mentioned in TSB #PIC5650M. [1, 3, ⭐ MANUFACTURER TSB — highest authority] Bulletin #PIC5650M
• B2955 — This code for 'Security Sensor Data Circuit Malfunction' is specifically mentioned in TSB PI1180A for 2014-2015 models. If seen with B3935 on a vehicle from that era, it strongly points to the need for a BCM software update.

Technical Service Bulletins (TSBs) & Recalls

• PIC5650M: Notes an intermittent no crank, no start, or stall concern with the security light on, associated with DTCs B3055, B3060, and B3935 among others. Lists numerous RFI sources like aftermarket LEDs and phone chargers. [⭐ MANUFACTURER TSB — highest authority] Bulletin #PIC5650M
• PIC5650J: Explicitly warns technicians not to replace any parts for this concern and to first investigate RFI from devices on the customer's keychain. [⭐ MANUFACTURER TSB — highest authority] Bulletin #PIC5650J

Platform-Specific Known Issues

• This platform is highly susceptible to Radio Frequency Interference (RFI) causing a no-start and setting code B3935, as documented in GM TSBs #PIC5650J and #PIC5650M.
• A known poor ground location on 2017-2018 K2XX models is G218, located behind the driver's side kick panel. Insulation can get caught under the ground nut, causing a poor connection and a wide range of electrical faults.

Mechanic-Grade Diagnostic Values

• Immobilizer Exciter Coil Resistance — expected: 5 - 20 Ohms. Failure: A reading of 0 Ohms (short) or infinite/OL (open circuit) indicates a failed coil.
• BCM Power Supply Voltage (Key-On) — expected: 12.0 - 12.6+ Volts. Failure: Voltage dropping below 12V during the key-on sequence (before cranking) can corrupt the authentication handshake.
• BCM Ground Circuit Resistance — expected: Less than 1.0 Ohm. Failure: A reading higher than a few ohms indicates a poor ground connection (e.g., at G218), which can cause intermittent module failure.

Hidden / Shadow Codes Worth Checking

• B3935 04: Transponder Authentication Failed. This is the most common symptom byte, indicating the key was read but the cryptographic challenge failed. (see via GM GDS2 or other professional scan tool capable of reading manufacturer-specific symptom bytes.)
• B3935 3A: Incorrect Component Installed. This byte often appears after a failed attempt to install a used or incorrect BCM or key without proper programming, as the security data does not match. (see via GM GDS2 or other professional scan tool capable of reading manufacturer-specific symptom bytes.)
• B3935 57: Transponder Authentication Error (calculation mismatch). This specifically means the TDM recognized the key but the challenge/response calculation between the key and the module did not match. (see via GM GDS2 or other professional scan tool capable of reading manufacturer-specific symptom bytes.)

Scan Tool Commands That Help

• GDS2 (Global Diagnostic System 2): Immobilizer Learn — Located under Module Diagnostics > Body Control Module > Configuration/Reset Functions. This procedure is required when replacing the BCM, ECM, or any ignition keys. It erases all previously learned keys and reprograms existing and new keys to the vehicle in a single session.
• GDS2 (Global Diagnostic System 2): Immobilizer/Theft Data List — Allows viewing of live data PIDs such as 'Immobilizer Challenge Status,' 'Key Status,' and 'Number of Learned Keys.' During a start attempt, 'Immobilizer Challenge Status' should transition to 'Pass.' If it shows 'Fail' or 'No Response,' it confirms the authentication failure in real-time.

Wiring & Ground Locations

• G218 — Behind the driver's side kick panel, near the floor. Applies to 2017-2018 K2XX models.. This is a major instrument panel ground that serves the Body Control Module (BCM). A loose or corroded connection here is a well-documented cause of various electrical faults, including immobilizer errors.
• G110 / G114 — On the front of the engine block or nearby frame rail on 2019-2020 T1XX models.. These are primary engine and body harness grounds. GM issued Engineering Information (EI) bulletin PIE0548 to investigate loose connections at these specific grounds on the T1XX platform, which can cause a wide array of electrical issues, including no-start conditions.
• Immobilizer Exciter Coil Connector — A small 2-pin connector attached to the immobilizer antenna ring around the ignition switch (keyed) or near the push-to-start button assembly.. A loose or corroded connection at this specific point will prevent the BCM from energizing or reading the key's transponder, directly causing codes like B3055, B3060, and B3935.

Real Owner Repair Stories

• Forum report on a similar K2XX platform (2016 Chevrolet Colorado) — Intermittent no-start, security light on, code B3935 stored.
  ❌ Tried (didn't work) Trying spare key, Replacing key fob battery, Checking for obvious RFI sources
  ✅ What actually fixed it The owner had recently installed an aftermarket LED bulb in the interior dome light. The cheap LED driver emitted enough RFI to disrupt the key authentication process. Removing the LED bulb permanently resolved the B3935 code.
• Forum report on gm-trucks.com (2015 GMC Sierra (K2XX Platform)) — Intermittent no-crank, no-start with code B3935.
  ❌ Tried (didn't work) Using spare key, Replacing vehicle battery, Isolating key from RFI sources
  ✅ What actually fixed it After extensive diagnosis, the problem was determined to be an internal fault in the Body Control Module (BCM). The final fix was replacing the BCM and having a dealer perform the 'Immobilizer Learn' procedure with GDS2.

OEM Part Supersession History

• 13580082 → 13529634, 13590048 (verify by VIN) — Part consolidation and minor internal revisions.
  Heads up: When programming new keys, all existing keys must be present. The 'Immobilizer Learn' procedure erases all keys from the BCM, and any key not present during the relearn process will be permanently de-registered from the vehicle.

Model Year Variations Within This Range

• 2019-2020 (T1XX Platform): These models use GM's new Vehicle Intelligence Platform (VIP) or 'Global B' architecture. While the B3935 code's meaning is the same, the BCM is more complex and has enhanced cybersecurity. Programming functions like 'Immobilizer Learn' require a modern GDS2 scan tool with an active internet connection for security handshakes with GM's servers.
• 2017-2018 (K2XX Platform): These models use the previous generation electrical architecture. While still complex, BCM and key programming procedures are slightly less stringent than on the newer VIP/Global B platform. Known issues like the G218 ground are specific to this K2XX platform.