B3935 on 2007-2010 Pontiac G5: Security Light, No-Start Causes and Fixes

What's Unique About the 2007-2010 Pontiac G5

The Pontiac G5 and its platform-mates (like the Chevrolet Cobalt, HHR, and Saturn Ion) are well-known for the sensitivity of their Passkey III+ (PK3+) immobilizer systems. General Motors issued specific Technical Service Bulletins (TSBs), including PIC5650L and PIC5650M, explicitly warning technicians *not* to replace parts for this code initially. Instead, the first step is always to investigate RFI from devices on the owner's keychain, a unique diagnostic instruction that highlights how common this specific issue is on this platform. The TSBs list a wide range of potential RFI sources, including other vehicle keys, toll passes, office access cards, and even aftermarket phone chargers or LED lights.

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 (no-start)
• Engine starts but stalls immediately after
• Security light is illuminated or flashing on the instrument cluster.
• A 'no crank, no start, or start stall concern with the security light coming on.' as stated in TSB #PIC5650M.
• In some cases, remote start may work, but the engine dies when the key is inserted and turned to the 'Run' position.

Most Likely Causes

1. Radio Frequency Interference (RFI) 🔴 High Probability As documented in GM TSBs #PIC5650L, #PIC5650M, and subsequent versions, the Passkey III+ immobilizer system is highly sensitive to other electronic devices. The 125 kHz frequency used by the system can be 'jammed' by other car fobs, office access cards, toll passes, aftermarket phone chargers, and even some aftermarket LED lights.
   How to confirm: Remove all other keys, fobs, and RFID cards from your keychain. Unplug any aftermarket chargers or accessories from the power outlets. Attempt to start the vehicle with only the single Pontiac G5 key. If the car starts reliably, RFI was the cause.
   Typical fix: Keep the ignition key isolated from other electronic devices. No parts are required.
   Est. part cost: $0
2. Damaged or Faulty Ignition Key 🟡 Medium Probability The transponder chip (often a Circle+ or PK3+ chip) embedded in the key head can fail over time or if the key is dropped. This prevents it from correctly responding to the car's security challenge, even if it is the correct physical key.
   How to confirm: Attempt to start the vehicle with a known-good spare key. If the spare key works consistently and the primary key does not (even when isolated), the primary key is faulty.
   Typical fix: The faulty key must be replaced. A new key will need to be cut and programmed to the vehicle by a dealer or an automotive locksmith. Some DIY programming kits are also available.
   Est. part cost: $50-$150
3. Worn Ignition Lock Cylinder ⚪ Low Probability The ignition lock cylinder houses the exciter coil (antenna) that reads the key's transponder. Heavy keychains can cause wear and tear on the cylinder and its internal connections over time, leading to intermittent reading failures. Corrosion or damage to the wiring for the coil can also cause this issue.
   How to confirm: After ruling out RFI and a bad key, a technician can test the resistance and signal from the exciter coil. Wiggling the key in the ignition may also intermittently cause the problem to appear or disappear. A visual inspection of the wiring harness at the back of the cylinder may reveal damage.
   Typical fix: Replace the ignition lock cylinder and perform a security relearn procedure.
   Est. part cost: $70-$200

Rare But Worth Checking

• Body Control Module (BCM) or Wiring Issue: This is very rare. TSB #PIC5650L specifically advises against replacing modules for this code unless all other causes are definitively ruled out. A wiring fault, such as a loss of power or ground to the Theft Deterrent Module/antenna, is more likely than an internal BCM failure. On other GM platforms, a software update for the BCM has been issued to fix this code when it appears with B2955, though this is not specifically documented for the G5.

Diagnosis Steps

1. Observe the symptoms: Confirm a no-crank or start-and-stall condition with the security light on.
2. Isolate the Ignition Key: Remove the G5 key from your keychain. Ensure no other keys, fobs, or access cards are nearby. Unplug any phone chargers or aftermarket accessories. Try to start the car. This is the most crucial first step.
3. Try the Spare Key: If isolating the key doesn't work, try your spare key. If the spare works, the original key is bad.
4. Perform a Security Relearn Procedure: 🎬 Watch this guide on the GM Passkey relearn procedure. Turn the key to the 'ON' position (without starting) and wait for the security light to turn off, which takes about 10-11 minutes. Turn the key off for 10 seconds. Repeat this process two more times for a total of three cycles and 30+ minutes. After the third cycle, turn the key off and then try to start the engine. This can sometimes resynchronize the system after a glitch or part replacement.
5. Inspect Fuses: Check fuses related to the ignition and Body Control Module (BCM) to rule out simple electrical issues. On the related Cobalt, a loss of B+ power to the theft deterrent antenna at the ignition switch was found to be the cause.
6. Professional Diagnosis: If the above steps fail, the issue is likely with the ignition lock cylinder, wiring, or BCM, and requires professional diagnosis with a scan tool that can read GM-specific body codes and check for symptom bytes like B3935 57 ('Transponder Authentication Error').

Parts You'll Likely Need

• Ignition Key with Transponder (OEM #23372322) — If the original key's internal transponder chip has failed, a new key is the only solution. This is the most common part needed if the problem is not RFI. The key uses a PK3+ or Chip ID 46 (Circle+) transponder.
  Trusted brands: ACDelco, GM
  OEM price range: $70-$120
  Aftermarket price range: $40-$80
• Ignition Lock Cylinder with Passlock Sensor (OEM #20869121) — If the antenna ring (exciter coil) that reads the key fails, the entire lock cylinder assembly often needs to be replaced. This is a less common but known failure point. The housing which contains the sensor is also available separately.
  Trusted brands: ACDelco, Dorman, Standard Motor Products
  OEM price range: $120-$200
  Aftermarket price range: $70-$150

Related Codes That Often Appear With This One

• B3055 — This is another GM immobilizer code, often for 'Key Not Present'. It is frequently triggered by the same RFI or key communication issues and is mentioned alongside B3935 in TSB #PIC5650M.
• B3060 — This code for 'Unprogrammed Key' can also be set during the same fault event, as the module may fail to recognize the key correctly. It is also cited in TSB #PIC5650M.

Technical Service Bulletins (TSBs) & Recalls

• PIC5650M: Mentions B3935 in relation to intermittent no-start/stall conditions with the security light on.
• PIC5650L: Explicitly warns against replacing parts for B3935 and points towards investigating Radio Frequency Identification interference first.
• PIC5650 (various revisions C, J, K, etc.): An ongoing diagnostic tip bulletin from GM that repeatedly emphasizes RFI from aftermarket devices (phone chargers, LED lights, toll passes) as the primary cause for intermittent security issues and codes like B3935.

Platform-Specific Known Issues

• Per TSB #PIC5650L, for an intermittent no crank/start with code B3935, technicians are advised: 'Technician should not replace any parts for this concern. If unable to duplicate the concern ask if the customer uses any Radio Frequency Identification Devices...'
• TSB #PIC5650M groups B3935 with several other security and chassis codes related to an 'intermittent no crank, no start, or start stall concern with the security light coming on.'
• A user on the DF Kit Car Forum building a race car from a 2009 G5 experienced this exact code and symptom after moving the ignition switch behind an aluminum dash panel, suspecting the metal was blocking the transponder signal.

Mechanic-Grade Diagnostic Values

• Vehicle System Voltage during Relearn — expected: Above 12.0 Volts. Failure: Voltage dropping below 12.0V can cause the 30-minute relearn procedure to fail.
• BCM Minimum Operating Voltage — expected: 9.0 Volts or higher. Failure: If system voltage drops below 9.0V during a crank attempt, the BCM may malfunction and set erroneous codes.

Hidden / Shadow Codes Worth Checking

• B3935 57: Transponder Authentication Error. This specific symptom byte confirms the BCM/TDM identified the key, but the cryptographic challenge-response calculation failed. This points more specifically to a faulty chip in the key or RFI, rather than an unknown key. (see via A professional GM-specific scan tool like a Tech 2 or GDS2 is required to view symptom bytes.)

Scan Tool Commands That Help

• GM Tech 2 / GDS2: Immobilizer Learn — This function is used to initiate the security relearn process after replacing the BCM, Theft Deterrent Module (TDM), or ignition keys.
• GM Tech 2 / GDS2: Add/Replace Keys — A specific function within the TDM/BCM menus to program additional keys to the vehicle's memory after the initial master key is learned.
• GM SPS (Service Programming System): Body Control Module Programming and Setup — Used to flash the BCM with new software. While not a common fix for the G5, on other GM platforms, a BCM software update was the specified fix for B3935, particularly when seen with other codes like B2955.

Wiring & Ground Locations

• Body Control Module (BCM) — In the passenger footwell, behind a removable panel on the side of the center console.. The BCM is the central computer for the Passkey III+ anti-theft system. All communication from the key and ignition cylinder is processed here.
• Theft Deterrent Module (TDM) — Located on the steering column, near or part of the ignition switch assembly.. This module contains the exciter coil (antenna) and communicates directly with the key's transponder before sending data to the BCM.
• G201 — Behind the left side of the dash, near the steering column.. This is a major instrument panel ground point. A poor connection here can affect the BCM, Instrument Panel Cluster (IPC), and the Theft Deterrent system.
• G203 — Behind the right side of the dash.. Another key instrument panel ground. Wiring diagrams show this as a ground point for various BCM functions.
• BCM Connector C2, Pin 62 — On the BCM located in the passenger footwell.. This pin corresponds to the WHITE with BLACK stripe wire from the Passlock system. Aftermarket bypass modules require cutting this wire, indicating it's a critical data line.

Real Owner Repair Stories

• Chevy HHR Network Forum User (Chevrolet HHR (platform mate)) — Security light stays on constantly, vehicle will not start. The 30-minute relearn procedure fails; the security light never turns off during the 10-minute cycles.
  ❌ Tried (didn't work) Standard 30-minute relearn procedure., Disconnecting the battery to reset modules.
  ✅ What actually fixed it The owner had attempted to repair damaged wiring to the theft deterrent system using butt splice connectors. It was suspected that the splices introduced too much resistance into the sensitive Passkey circuit, causing the relearn to fail. Properly soldering and heat-shrinking the wires would be the correct repair to eliminate resistance as a variable.
• YouTube User Comment (GM Truck (Silverado/Sierra platform)) — Multiple intermittent electrical issues, including ABS light, check engine light, and low voltage readings, which can create an environment for security codes like B3935 to set.
  ❌ Tried (didn't work) Chasing individual electrical gremlins without success.
  ✅ What actually fixed it The user discovered the main ground strap from the chassis to the back of the engine block was corroded and failing. Replacing this single ground strap resolved all the intermittent electrical problems by stabilizing the vehicle's entire ground system.

OEM Part Supersession History

• 15282624, 15293892, 15785084, 15825311, 15872667, 15918809, 19177419, 19177836, 19207816, 22707216, 22721458, 25806988, 25848835, 25848845, 25979925, 89024348, 89024362, 89024364 → 20869121 — Part consolidation and design updates by GM over many model years.
  Heads up: This is an uncoded lock cylinder. It must be keyed to match the existing keys and requires a security relearn procedure after installation.