P1628

What Does Code P1628 Mean?

The diagnostic trouble code P1628, "Module Ignition Supply Input Malfunction," indicates that the Powertrain Control Module (PCM), also commonly referred to as the Engine Control Module (ECM), has detected an irregularity or an improper voltage level on one of its dedicated ignition supply input circuits. This circuit provides the PCM with a critical signal indicating the state of the vehicle’s ignition switch, typically distinguishing between "OFF," "ACC," "RUN," and "START." The PCM relies on this input to properly activate internal components, manage power distribution to various sensors and actuators, enable fuel delivery, control ignition timing, and initiate system diagnostics (e.g., Key On, Engine Off (KOEO) and Key On, Engine Running (KOER) self-tests). A "malfunction" signifies that the voltage observed by the PCM on this input does not correlate with the expected ignition switch position, or it is intermittent, too low, too high, or absent when it should be present. This discrepancy prevents the PCM from performing its functions correctly, often leading to a no-start condition or erratic engine operation as the module cannot reliably determine if the engine should be running or prepare to run.

Common Symptoms

• Engine Cranks But Won’t Start: The PCM may not enable fuel injection or ignition spark due to an incorrect ignition state signal.
• Complete No-Start Condition: The starter motor may not engage if the PCM inhibits starting in response to the fault.
• Intermittent Engine Stalling: A momentary loss or fluctuation in the ignition supply signal while the engine is running can cause it to abruptly shut off.
• Illuminated Malfunction Indicator Lamp (MIL): The Check Engine Light will be illuminated.
• Other Warning Lights: Depending on the vehicle, battery light, security light, or other system warnings might activate.
• Inoperative Accessories: Certain vehicle accessories or dashboard indicators may fail to operate correctly.
• Erratic Engine Operation: If the signal is intermittent, it can lead to rough idling, misfires, or poor acceleration.

What Causes the Code P1628?

• Damaged or Corroded Wiring: Open circuits, shorts to ground or power, or excessive resistance in the ignition supply wire(s) leading to the PCM.
• Faulty Ignition Switch: Internal wear or failure of the ignition switch preventing it from reliably providing the correct voltage output to the PCM in different key positions.
• Blown Fuse: A fuse protecting the ignition switch circuit, PCM power supply, or related ignition relays may be open.
• Defective Ignition Relay: If an ignition relay is part of the power supply circuit to the PCM, a faulty relay can cause this code.
• Corroded Battery Terminals or Weak Battery: Poor overall system voltage or intermittent power delivery can sometimes trigger false P1628 codes, although less direct.
• Internal PCM/ECM Failure: Although less common, an internal fault within the PCM’s input monitoring circuitry can cause this code to set.

How to Diagnose and Troubleshoot

Diagnosing P1628 requires systematic electrical testing, often involving a digital multimeter (DMM) and a comprehensive OBD-II scanner capable of monitoring live data and freeze frame information.

1. Retrieve and Analyze DTCs and Freeze Frame Data: Connect an OBD-II scanner to confirm P1628 and check for any related or accompanying codes (e.g., U-codes indicating communication issues, other power supply codes). Analyze freeze frame data to note engine conditions (RPM, voltage, load) when the code was set, which can provide clues about the failure mode (e.g., intermittent vs. continuous).
2. Perform a Thorough Visual Inspection: Carefully inspect the battery terminals and cables for corrosion or looseness. Examine the vehicle’s fuse box(es) and check all fuses related to the ignition system, PCM power, and engine control. Visually inspect the wiring harness from the ignition switch to the PCM for any signs of chafing, cuts, pinches, or corrosion, especially at connectors. Pay close attention to the PCM’s main power connector.
3. Test Ignition Switch Output: Using a DMM, back-probe the output wire(s) from the ignition switch that supply power to the PCM. With the key in the "RUN" or "ON" position, verify that a consistent battery voltage (B+) is present. Cycle the ignition switch several times to check for intermittency. Compare readings to vehicle-specific wiring diagrams and specifications.
4. Verify PCM Ignition Input Voltage: Locate the specific pin(s) on the PCM connector that receive the ignition supply input. With the PCM connector disconnected and the key in the "RUN" position, test for battery voltage at the wire leading to that pin. Then, carefully back-probe the connector with it attached to the PCM (if feasible and safe) to verify the voltage the PCM is actually receiving.
5. Conduct Continuity and Resistance Checks: With the battery disconnected, use a DMM to perform continuity checks on the ignition supply wire(s) between the ignition switch (or relevant relay) and the PCM connector. Check for opens or shorts to ground or power. Also, measure the resistance of the circuit; excessively high resistance indicates a potential problem.
6. Test Relays (If Applicable): If the ignition supply to the PCM passes through a relay (e.g., an ignition relay or main relay), test the relay for proper operation. Check the coil resistance, and verify continuity through the power contacts when the coil is energized.
7. Perform a "Wiggle Test": With the DMM connected and monitoring the relevant voltage input at the PCM, gently wiggle the wiring harnesses and connectors, especially those associated with the ignition switch and the PCM. Look for any fluctuations in the voltage reading that could indicate an intermittent connection.

Recommended Repairs and Solutions

Once the root cause of P1628 has been identified through systematic diagnosis, the appropriate repair can be performed.

• Repair or Replace Damaged Wiring: If damaged, corroded, or open circuits are found in the wiring harness, the affected section should be repaired using appropriate automotive-grade connectors and wiring, or the entire harness section replaced if damage is extensive. Always ensure proper soldering and heat-shrink tubing for environmental protection.
• Replace the Ignition Switch: If the ignition switch is determined to be faulty and not consistently providing the correct voltage output, it should be replaced with an OEM-quality or equivalent aftermarket unit.
• Replace Blown Fuses and Investigate Cause: If a blown fuse is discovered, replace it with a fuse of the correct amperage rating. It is crucial to investigate why the fuse blew, as it often indicates an underlying short circuit or overload that needs to be resolved to prevent immediate re-occurrence.
• Replace Faulty Ignition Relays: If an ignition-related relay is found to be defective after testing, replace it. Ensure the replacement relay matches the original specifications.
• Clean Battery Terminals and Cables: If corrosion or poor connections were found at the battery, thoroughly clean the terminals and cable ends. Ensure all connections are tight to guarantee stable system voltage.
• PCM/ECM Replacement (As a Last Resort): Only after all other potential causes have been thoroughly investigated, confirmed, and ruled out, should PCM replacement be considered. This is an expensive and often complex repair that typically requires programming (flashing) the new module to the vehicle’s specific VIN and options. An incorrect diagnosis can lead to unnecessary cost and continued issues.