P1660

What Does Code P1660 Mean?

The diagnostic trouble code P1660, generally described as “Output Circuit Check Signal High,” indicates that the Powertrain Control Module (PCM), also commonly referred to as the Engine Control Module (ECM), has detected an abnormally high voltage on a dedicated feedback or monitoring circuit associated with one of its controlled output drivers. The PCM continually performs internal diagnostics and monitors the integrity of circuits controlling various actuators and components (e.g., fuel injectors, ignition coils, solenoids, relays). These monitoring circuits provide the PCM with real-time feedback regarding the operational status or voltage present at the output of its driver circuits. When the PCM observes a voltage on this check signal line that significantly exceeds the expected operational parameters for a “high” condition, typically indicating a short to battery voltage or an open feedback loop presenting an uncontrolled high impedance state, it sets the P1660 code. Because P16xx codes are manufacturer-specific, the exact output circuit this code refers to can vary widely between vehicle makes and models. It could relate to a fuel pump monitor circuit, an EGR position sensor feedback, a specific solenoid control circuit, or another critical system where the PCM expects a precise feedback voltage to confirm correct operation.

Common Symptoms

• Check Engine Light (MIL) illumination: This is almost always the first and most direct symptom.
• Engine performance issues: Depending on the affected output circuit, symptoms may include rough idle, misfires, reduced engine power, hesitation, or stalling.
• No-start or hard-start condition: If the affected output circuit controls a critical component like the fuel pump or ignition system.
• Erratic operation of a specific component: For instance, a fuel pump running continuously or not at all, an EGR valve stuck open or closed, or other components operating outside normal parameters.
• Transmission shifting irregularities: If the code pertains to a transmission solenoid output circuit.
• Inoperable ancillary systems: Such as cruise control, air conditioning, or cooling fans, if the output circuit is related to these systems.

What Causes the Code P1660?

• Internal PCM driver fault: A common cause for P16xx codes, where the electronic driver (transistor or integrated circuit) within the PCM responsible for controlling the output circuit has failed internally, causing an erroneous high voltage feedback signal.
• Wiring harness issues: A short circuit to battery voltage (B+) on the specific feedback or monitoring wire leading to the PCM, or an open circuit in the feedback line, which can result in an uncontrolled high voltage state due to impedance.
• Faulty actuator or component: The component being controlled by the output circuit (e.g., a relay, solenoid, sensor providing feedback) may have an internal short or fault that directly supplies an unintended high voltage back to the PCM’s monitoring circuit.
• Corrosion or poor connection: Degraded or loose electrical connections at the PCM connector or at the connector of the specific component, leading to intermittent or continuously erroneous high voltage readings.

How to Diagnose and Troubleshoot

Diagnosing P1660 requires meticulous attention to detail and access to vehicle-specific service information. Given its manufacturer-specific nature, the first critical step is to consult the vehicle’s factory service manual to identify precisely which “output circuit” P1660 pertains to for the specific make, model, and year.

1. Perform a Thorough Visual Inspection: Begin by visually inspecting the wiring harness and connectors associated with the suspected output circuit (once identified from the service manual) and the PCM. Look for any signs of chafing, bare wires, corrosion, loose terminals, or obvious damage. Pay close attention to areas where the harness passes through firewalls or near hot engine components.
2. Utilize an OBD-II Scan Tool:
   • Connect a professional-grade scan tool to confirm the presence of P1660.
   • Check for any other related diagnostic trouble codes (DTCs) that might offer more specific clues about the affected subsystem.
   • Monitor live data parameters related to the suspected output circuit. While direct “check signal” feedback might not always be a broadcast PID, look for parameters that would be affected by the output circuit’s malfunction (e.g., fuel pressure, EGR position, solenoid commanded state vs. actual state).
   • Perform active tests (bidirectional control) on the suspected output component if available, while monitoring feedback or circuit voltage with a DMM.
3. Digital Multimeter (DMM) Testing at the PCM Connector:
   • With the ignition OFF and the PCM connector disconnected, consult the vehicle’s wiring diagram for the specific pin-out of the “Output Circuit Check Signal” wire at the PCM.
   • Resistance Test: Check the resistance of the suspect wire from the PCM connector back to the component. Compare to specifications. Look for open circuits or unintended low resistance paths.
   • Continuity Test (to B+ and Ground): With the PCM disconnected and all other connectors to the component disconnected, test for continuity between the signal wire and battery positive (+) and battery ground (-). Any continuity to B+ indicates a short to voltage, and continuity to ground indicates a short to ground. There should be no continuity to either unless specified otherwise.
   • Voltage Test (with PCM connected, back-probing): Carefully back-probe the PCM connector at the identified signal wire (with the PCM connected and ignition ON, engine OFF or running, as appropriate). Compare the measured voltage to the specified reference voltage in the service manual. An abnormally high voltage, particularly above battery voltage or higher than the specified operational feedback, confirms the P1660 condition on that line.
   • Voltage Drop Test: If a relay or high-current component is involved, perform voltage drop tests across the power and ground sides of the circuit to ensure proper current flow and no excessive resistance.
4. Component-Specific Testing: If the DMM tests suggest external wiring or component issues, proceed to test the specific actuator or sensor providing the feedback. This may involve checking its internal resistance, power, and ground supplies, or its own output signal directly at its connector.
5. Wiggle Test: With the ignition ON and observing live data or the DMM, gently wiggle sections of the wiring harness suspected of having intermittent issues. Watch for fluctuations in voltage or changes in the fault condition.

Recommended Repairs and Solutions

Once the root cause of the P1660 code has been accurately diagnosed, the following repairs and solutions are commonly implemented:

• Repair or Replace Damaged Wiring: If the diagnosis points to a short to voltage or an open circuit in the wiring harness, the damaged section should be professionally repaired or the entire sub-harness replaced. Ensure all repairs are made using proper automotive-grade wiring and connectors, with sealed splices to prevent future corrosion.
• Replace Faulty Actuator or Component: If the component attached to the output circuit (e.g., a solenoid, relay, or sensor) is found to be internally shorted or faulty and is causing the “high” signal condition, it must be replaced with a new, OE-equivalent part.
• Clean or Repair Corroded/Loose Connectors: Address any signs of corrosion at electrical connectors by cleaning the terminals thoroughly. If terminals are bent, pushed out, or corroded beyond repair, the connector housing or individual terminals should be replaced. Apply dielectric grease to prevent future corrosion.
• PCM Replacement and Reprogramming: If all external wiring and components have been rigorously tested and confirmed to be in good working order, and the internal PCM driver is unequivocally identified as the source of the fault, then PCM replacement may be necessary. This is often the most expensive solution and should only be pursued after exhausting all other diagnostic avenues. After replacement, the new PCM will require programming or flashing to the vehicle’s specific VIN and calibration file.

Mechanic’s Tip: Always prioritize diagnosis based on the vehicle’s specific service information for P16xx codes. These codes are proprietary and often have unique diagnostic flowcharts provided by the manufacturer. After any repair, clear the DTCs from the PCM and perform a comprehensive drive cycle to ensure the code does not return and that all system monitors have successfully run and passed.