P1620

What Does Code P1620 Mean?

Diagnostic Trouble Code P1620 typically indicates an issue within the Theft Deterrent System (TDS) fuel enable circuit, signifying that the Powertrain Control Module (PCM) or Engine Control Module (ECM) is not receiving a valid fuel enable signal from the anti-theft module. This signal, often referred to as the PassLock, VATS (Vehicle Anti-Theft System), or Immobilizer signal, is a critical security handshake that authorizes the PCM to activate the fuel injectors and ignition system. The PCM detects this problem when it fails to receive the expected serial data message or a specific voltage signal from the anti-theft module during engine cranking or operation. If this signal is absent, incorrect, or intermittent, the PCM interprets it as an unauthorized attempt to start the vehicle or a system malfunction. Consequently, the PCM often disables fuel delivery, preventing the engine from starting or running, and sets the P1620 code to indicate this security system failure.

Common Symptoms

• Engine cranks but does not start: This is the primary symptom, as the PCM inhibits fuel injection and/or spark when the fuel enable signal is missing.
• Engine starts briefly (e.g., 2-3 seconds) then immediately stalls: In some vehicle applications, the engine may fire momentarily before the PCM fully disables fuel due to the invalid security signal.
• Security light illuminated or flashing on the dashboard: The anti-theft system warning light often activates in conjunction with P1620.
• No audible fuel pump prime upon ignition ON: The fuel pump relay may not be commanded ON by the PCM if the security system is preventing engine operation.
• Loss of communication with the anti-theft module (less common direct symptom): While not a direct symptom of P1620, issues preventing the fuel enable signal can sometimes manifest with related communication errors.

What Causes the Code P1620?

• Faulty Theft Deterrent Module (TDM) or Immobilizer Control Module: An internal malfunction within the module responsible for generating and sending the fuel enable signal to the PCM.
• Open or short circuit in the fuel enable signal wire: Damaged, corroded, or disconnected wiring between the TDM/Immobilizer module and the PCM can interrupt the signal path.
• Corrupted serial data communication: Problems on the vehicle’s data bus preventing proper communication between the anti-theft module and the PCM.
• Faulty Powertrain Control Module (PCM) or Engine Control Module (ECM): Although less frequent, a defective PCM input circuit or internal logic may fail to correctly interpret the fuel enable signal.
• Improperly programmed or mismatched modules: If the TDM/Immobilizer or PCM has been replaced without proper security relearn procedures or programming, modules may not synchronize.
• Ignition switch cylinder malfunction (VATS/PassKey systems): Issues with the ignition key reader or resistor pellet in the key itself can prevent the anti-theft module from receiving the correct key ID, thus not sending the fuel enable signal.

How to Diagnose and Troubleshoot

Diagnosing P1620 requires a specialized approach, focusing on the communication and signal integrity of the vehicle’s anti-theft system:

1. Retrieve and Document Freeze Frame Data: Connect an OBD-II scan tool and record any freeze frame data associated with P1620. This data provides crucial operational parameters at the time the code was set, which can help differentiate between a starting-related issue and an intermittent fault while running. Clear the code after documentation.
2. Scan for Related Anti-Theft Codes: Use a scan tool capable of communicating with all vehicle modules (PCM, BCM, TDM/Immobilizer) to check for additional anti-theft, communication, or body control module (BCM) codes. These can provide context or indicate a broader system failure.
3. Verify Fuel Enable Signal (If Applicable):
   • For dedicated wired systems (e.g., older GM VATS): Consult the vehicle’s wiring diagrams to locate the specific fuel enable signal wire between the anti-theft module and the PCM. Use a digital multimeter (DMM) or an oscilloscope to monitor the voltage on this wire while attempting to crank the engine. The signal should typically show a pulsed voltage or a steady voltage (e.g., 5V or 12V) that indicates an “enable” state.
   • For serial data-based systems (e.g., PassLock, immobilizer): Use an advanced scan tool to access live data PIDs for “Fuel Enable Status,” “Immobilizer Status,” or “Security Status.” Monitor these parameters while attempting to start the engine, expecting a “Yes,” “Enabled,” or “Recognized” status.
4. Inspect Wiring and Connectors: Perform a thorough visual inspection of the wiring harness and connectors connecting the TDM/Immobilizer module, BCM (if applicable), and PCM. Look for signs of corrosion, chafing, loose terminals, or aftermarket modifications (e.g., remote start installations). Pay particular attention to the data bus lines (CAN, Class 2) and the dedicated fuel enable wire if present. Perform a gentle wiggle test on relevant connectors while monitoring the fuel enable signal or attempting to start.
5. Test Power and Ground Circuits for TDM/Immobilizer: Using a DMM, verify that the anti-theft module receives proper battery voltage and has a solid ground connection. A lack of either can prevent the module from operating correctly and sending the necessary signal.
6. Perform a Security Relearn Procedure: If no obvious wiring or module failures are identified, and especially after module replacement, attempt a security relearn procedure as specified by the manufacturer. This often involves specific ignition key cycles or programming steps with a scan tool to synchronize the anti-theft module with the PCM.
7. PCM Integrity Check: If all other components and circuits are confirmed functional, and the fuel enable signal is verified as correct at the PCM input, then the PCM itself may be faulty. Confirm all PCM power and ground circuits are intact before considering PCM replacement.

Recommended Repairs and Solutions

Once the root cause for P1620 has been precisely identified, the following repair actions are commonly undertaken:

• Repair or Replace Damaged Wiring: If the diagnostic process reveals an open circuit, short, or excessive resistance in the fuel enable signal wire or related data communication lines, the affected wiring section must be repaired or replaced using appropriate gauge wire, heat-shrink tubing, and secure connectors.
• Replace the Theft Deterrent Module (TDM) or Immobilizer Control Module: If the anti-theft module is determined to be faulty and unable to transmit the correct fuel enable signal, it will need to be replaced. Following replacement, the new module typically requires programming and/or a security relearn procedure using an OEM-level or advanced aftermarket scan tool to synchronize it with the vehicle’s PCM and VIN.
• Perform a Security Relearn Procedure: After replacing any component of the anti-theft system (TDM, PCM, or sometimes the ignition switch), a security relearn or programming procedure is almost always required. This process is critical for the vehicle’s security modules to properly communicate and authorize engine operation. Always follow the specific, detailed instructions provided in the vehicle’s service manual.
• Replace the Ignition Switch Cylinder or Key: In vehicles using VATS or PassKey systems, if the resistance pellet in the ignition key or the ignition lock cylinder’s resistor reading mechanism is defective, it can prevent the anti-theft module from receiving the correct key identification. Replacement of the cylinder, and potentially the key, with the correct resistance value is necessary.
• Replace the Powertrain Control Module (PCM): If all other components and wiring are thoroughly checked and verified as functional, and the PCM is conclusively diagnosed as the fault (e.g., unable to process a correct fuel enable signal), then PCM replacement is the solution. This will also necessitate subsequent programming, including a security relearn procedure, to properly integrate the new module into the vehicle’s system.

Mechanic’s Tip: Always be vigilant for aftermarket remote start or alarm systems. These can sometimes improperly integrate with or bypass the factory anti-theft system, leading to P1620 codes. Inspecting for non-OEM wiring splices or control modules should be part of the diagnostic routine. Additionally, consult vehicle-specific Technical Service Bulletins (TSBs) as P1620 often has manufacturer-specific causes and recommended fixes, particularly in GM vehicles where this code is prevalent.