P1907

What Does Code P1907 Mean?

DTC P1907 indicates a fault within the electrical circuit of the Kickdown Hold Relay, specifically detecting an open circuit or a short circuit to ground. The Powertrain Control Module (PCM) or Transmission Control Module (TCM), depending on vehicle architecture, monitors the electrical integrity of this relay’s control and/or load circuit. A kickdown hold relay is a component of the automatic transmission control system responsible for optimizing gear selection during aggressive acceleration demands. When the accelerator pedal is fully depressed (kickdown), the transmission typically downshifts to a lower gear to maximize engine RPM and power output. The “hold” function, often associated with performance or towing modes, implies maintaining a specific gear or preventing premature upshifts to ensure sustained power delivery. The PCM/TCM detects an abnormal voltage or resistance reading on the relay circuit that deviates from its expected operating parameters, signaling either an interruption in the circuit (open) or an unintended path to ground (short), preventing the relay from activating or deactivating as commanded, thereby affecting the transmission’s kickdown or gear-hold strategy.

Common Symptoms

• Lack of Kickdown Function: The transmission may not downshift aggressively when the accelerator pedal is fully depressed, resulting in sluggish acceleration.
• Delayed or Erratic Shifts: The transmission might exhibit unusual shifting patterns, including delayed upshifts or harsh downshifts, as the control module compensates for the non-functional relay.
• Reduced Engine Performance: Due to improper gear selection, the engine may operate outside its optimal power band, leading to a noticeable decrease in acceleration and overall vehicle performance.
• Illuminated Check Engine Light (MIL): The Malfunction Indicator Lamp will illuminate on the dashboard to alert the driver of the detected fault.
• No specific “Hold” Function Engagement: If the relay is part of a specific performance or towing hold function, that function may be inoperative.

What Causes the Code P1907?

• Faulty Kickdown Hold Relay: The relay itself may have failed internally due to burnt contacts, an open coil, or an internal short, preventing it from switching on or off correctly.
• Damaged Wiring Harness: An open circuit (e.g., a broken wire, corroded terminal) or a short circuit to ground within the relay’s control or load wiring harness. This could be due to chafing, corrosion, or physical damage.
• Corroded or Loose Connectors: Poor electrical connection at the relay socket or at any inline connectors within the circuit can lead to high resistance or intermittent open/short conditions.
• Blown Fuse: A blown fuse protecting the power supply circuit to the kickdown hold relay can cause an open circuit condition. This often indicates an underlying short circuit that caused the fuse to blow.
• Faulty Powertrain Control Module (PCM) or Transmission Control Module (TCM): While less common, an internal fault within the PCM/TCM’s driver circuit responsible for controlling the kickdown hold relay can lead to this code.

How to Diagnose and Troubleshoot

Diagnosis of P1907 requires a methodical approach, utilizing a digital multimeter (DMM) and a scan tool:

1. Initial Scan Tool Inspection: Connect an OBD-II scan tool to confirm P1907 is present. Check for any related transmission codes or voltage/communication issues. Review freeze frame data to understand the operating conditions when the code was set.
2. Locate the Kickdown Hold Relay: Refer to the vehicle’s service manual to precisely locate the kickdown hold relay, which is typically found in the under-hood fuse box, an interior fuse box, or integrated into the transmission control module’s relay bank.
3. Visual Inspection:
   • Inspect the relay itself for signs of physical damage, overheating, or corrosion.
   • Carefully examine the relay socket and its terminals for corrosion, bending, or looseness.
   • Trace the wiring harness leading to and from the relay. Look for any signs of chafing, cuts, bare wires indicating a short to ground, or stretched/broken wires indicating an open circuit. Pay close attention to areas where the harness passes through firewalls or near hot engine components.
   • Check the fuse(s) associated with the relay’s power supply circuit. If a fuse is blown, replace it but understand that an underlying short likely caused the blow.
4. Relay Functionality Test:
   • With the relay removed, use a DMM to test the resistance across the relay’s control coil (typically pins 85 and 86). Compare the reading to service manual specifications; an open circuit (infinite resistance) or a short (near zero resistance) indicates a faulty coil.
   • Apply fused battery voltage and ground to the control coil terminals (85 and 86) while checking for continuity across the switch terminals (30 and 87). The relay should audibly click, and continuity should be present when activated. No click or no continuity indicates a faulty relay. Alternatively, swap the relay with a known good, identical relay from another non-critical circuit if available for a quick functional check.
5. Circuit Integrity Test (with DMM):
   • Power Supply Check: With the relay removed and the ignition ON, use the DMM to check for battery voltage at the relay socket’s power input terminal (typically pin 30 or the fused power supply). If no voltage, trace the power wire back to its source, checking for open circuits or a blown fuse.
   • Control Circuit Check: Identify the PCM/TCM control wire for the relay. With the ignition ON, use a DMM to check for the proper control signal (voltage or ground) at the appropriate relay socket terminal (typically 85 or 86). This may require back-probing the connector while commanding the kickdown function with a bi-directional scan tool or observing the signal during a test drive. An absence of the expected signal indicates a problem with the PCM/TCM’s driver or the wiring to it.
   • Output Circuit Check: Check for continuity from the relay socket’s output terminal (typically pin 87) to the transmission component (e.g., solenoid, valve) it is designed to power. Infinite resistance indicates an open circuit.
   • Short to Ground Test: Disconnect the vehicle’s battery. With the relay removed, use the DMM to check for continuity between each of the relay socket’s terminals and a known good chassis ground. There should be an open circuit on all power and output lines; a very low resistance reading indicates a short to ground on that specific wire.
6. PCM/TCM Driver Test: If the relay and all associated wiring test good, the fault may lie within the PCM/TCM’s internal relay driver circuit. This typically requires specialized diagnostic equipment or replacement of the module for definitive confirmation.

Recommended Repairs and Solutions

Once the root cause of the P1907 code has been precisely identified through the diagnostic steps, the following repairs are typically recommended:

• Replace the Kickdown Hold Relay: If the relay fails internal testing (open coil, shorted contacts, or no switching action), replacement with an OEM quality relay is the most common solution.
• Repair or Replace Damaged Wiring: If an open circuit or short to ground is found in the wiring harness, the damaged section should be carefully repaired using appropriate automotive-grade wiring and connectors, or the entire section of the harness replaced if damage is extensive. Ensure all repairs are properly insulated and secured to prevent future issues.
• Clean and Secure Connectors: If corrosion or loose terminals are found at the relay socket or other connectors, clean them thoroughly with electrical contact cleaner and ensure a tight, secure connection. Replace terminals if they are too damaged or spread.
• Replace Blown Fuses: If a blown fuse was identified, replace it. However, it is paramount to determine and rectify the underlying cause of the blown fuse (usually a short circuit) before installing a new one, as it will likely blow again immediately if the short is not fixed.
• PCM/TCM Replacement: In rare cases where the PCM/TCM is determined to be the culprit, replacement and reprogramming (if required by the vehicle manufacturer) will be necessary. This should only be considered after all other possibilities have been exhaustively ruled out.

After any repair, clear the diagnostic trouble code using a scan tool and perform a thorough test drive to confirm the repair has resolved the issue and the code does not return. Pay close attention to transmission shift quality and the functionality of the kickdown feature.