P1661

What Does Code P1661 Mean?

The diagnostic trouble code P1661, often defined as “Output Circuit Check Signal Low,” indicates that the Powertrain Control Module (PCM), also referred to as the Engine Control Module (ECM), has detected an anomaly in a specific output circuit’s feedback loop. The PCM commands various actuators, relays, or solenoids to perform essential powertrain functions. To verify the successful execution of these commands and the electrical integrity of the associated circuit, the PCM incorporates a monitoring circuit. This circuit is designed to observe a feedback voltage signal, which should fall within a predefined operational range when the output is active. When the PCM identifies that this feedback signal voltage is consistently or intermittently below the manufacturer’s specified calibrated threshold, it interprets this as a fault and registers P1661. This condition suggests that the commanded output device may not be activating correctly, the circuit providing the feedback is compromised (e.g., an open circuit, high resistance, or a short to ground), or there is an internal fault within the output driver or monitoring logic of the PCM itself. The specific output affected by P1661 can vary significantly by vehicle manufacturer and model, often pertaining to auxiliary powertrain functions not covered by more generic P0xxx codes.

Common Symptoms

• Malfunction Indicator Lamp (MIL) Illumination: The “Check Engine” light will be illuminated on the dashboard.
• Intermittent or Consistent Engine Performance Issues: Depending on the affected output circuit, symptoms could range from rough idle, stalling, reduced power, or transmission shifting irregularities.
• Failure of a Specific System: The component controlled by the problematic output circuit may not function. For example, if it’s a fuel pump relay feedback circuit, the fuel pump may not activate. If an emissions solenoid, emissions system issues might arise.
• No-Start Condition: If the affected output circuit controls a critical component like the fuel pump or ignition system, a no-start condition could occur.
• Increased Fuel Consumption: If the faulty circuit relates to fuel management or air-fuel ratio control, fuel economy may suffer.

What Causes the Code P1661?

• Faulty Wiring or Connectors: An open circuit, short to ground, or high resistance in the wiring harness pertaining to the output device’s feedback signal circuit. This includes corroded terminals, damaged insulation, or loose connections.
• Defective Output Device: The relay, solenoid, or actuator itself may be internally faulty, preventing it from drawing the correct current or providing the expected feedback voltage. This could be due to internal winding issues or mechanical failure.
• Failed Driver Module or Relay: If the PCM controls a separate driver module (e.g., for a fuel pump or an auxiliary pump) or a dedicated relay, that module or relay could be defective, failing to activate or provide proper feedback.
• Internal PCM/ECM Fault: While less common, an internal failure within the PCM’s output driver circuit or the feedback monitoring circuit can erroneously set this code.
• Poor Ground Connection: An inadequate ground path for the output device or the PCM can lead to incorrect voltage readings and trigger this code.

How to Diagnose and Troubleshoot

Diagnosing P1661 requires a methodical approach, utilizing an OBD-II scanner, a digital multimeter (DMM), and visual inspection.

1. Retrieve Freeze Frame Data: Connect an OBD-II scanner and record all freeze frame data associated with P1661. This data captures engine parameters at the moment the code was set and can provide valuable clues regarding operating conditions. Clear the code after recording.
2. Identify the Affected Circuit: Consult the vehicle-specific service manual to determine which specific output circuit P1661 refers to for your make and model. This is critical as P16xx codes are often manufacturer-specific.
3. Visual Inspection:
   • Carefully inspect the wiring harness and connectors associated with the identified output device and its feedback circuit. Look for signs of chafing, cuts, corrosion, loose terminals, or heat damage.
   • Inspect the output device itself (relay, solenoid, actuator) for any visible damage or signs of overheating.
   • Check for any obvious aftermarket modifications that might interfere with circuit integrity.
4. Continuity and Resistance Checks (Key Off):
   • Disconnect the PCM connector and the connector for the specific output device.
   • Using a DMM, perform a continuity check on the feedback wire between the output device connector and the PCM connector. There should be continuity (near 0 ohms).
   • Check for shorts to ground on the feedback wire by measuring resistance between the wire and a known good chassis ground. Resistance should be infinite (open circuit).
   • Check for shorts to battery voltage on the feedback wire.
   • Measure the resistance of the output device itself (e.g., solenoid coil) if accessible and compare to specifications. An open circuit or excessively high resistance indicates an internal fault.
5. Voltage Checks (Key On/Engine Off & Engine Running):
   • With the relevant connectors reconnected (except possibly the PCM if advised by service manual for specific tests), use the DMM to measure voltage at the output device’s feedback pin.
   • If the PCM commands the output, verify that the expected voltage change occurs at the feedback pin. For example, if the PCM grounds a control circuit and expects a pull-up voltage, verify the presence of this voltage.
   • If the service manual specifies a dedicated feedback line, monitor its voltage directly while the PCM commands the output. A “Low” signal indicates it’s below the expected threshold.
   • If the output device is a relay, check for proper power and ground supply to the relay coil, and then check for switched power at its load side when activated.
6. Active Test with Scan Tool: If your scan tool supports bi-directional control, attempt to activate the problematic output device. While activating, monitor the feedback signal (if available as a PID) on the scan tool or manually measure voltage with a DMM. This helps confirm the PCM’s command and the circuit’s response.
7. PCM Terminal Voltage Check: If all external wiring and components test good, it may be necessary to backprobe the PCM connector at the specific feedback pin while the circuit is active. Compare the reading to service manual specifications. If the external circuit is good but the PCM input is low, it points to an internal PCM fault.

Recommended Repairs and Solutions

Once the root cause of P1661 has been identified through systematic diagnosis, the appropriate repairs can be performed:

• Repair or Replace Wiring/Connectors: The most common fix involves repairing damaged wiring, cleaning corroded terminals, or replacing faulty connectors. Ensure all repairs utilize proper automotive-grade wiring and soldering techniques, or use appropriate crimp connectors.
• Replace the Faulty Output Device: If the relay, solenoid, or actuator itself is determined to be internally defective (e.g., open coil, seized mechanism), replace it with an OEM-quality equivalent part.
• Replace Driver Module/Relay: If the issue lies with an intermediate driver module or a primary relay that the PCM controls, replace that component. Always verify power and ground to the new component before assuming it’s the sole fix.
• Address Grounding Issues: Clean and tighten any loose or corroded ground connections related to the circuit or the PCM. Ensure proper ground strap integrity.
• PCM Replacement/Reprogramming: If all other components and wiring have been thoroughly tested and confirmed to be in good working order, and the PCM terminal voltage check confirms a low signal at the PCM’s input despite correct input from the circuit, an internal PCM fault is indicated. In such cases, the PCM may need to be replaced and subsequently programmed to the vehicle. This should always be a last resort after exhaustive troubleshooting.

Mechanics’ Tips:

• Always start with the simplest checks, such as visual inspection and basic voltage checks, before delving into complex continuity tests.
• Consult specific wiring diagrams for the vehicle year, make, and model as P1661’s specific application varies greatly. Incorrectly identifying the circuit will lead to misdiagnosis.
• Pay close attention to any TSBs (Technical Service Bulletins) or recalls that may exist for the specific P1661 code or related systems for the vehicle.
• After any repair, clear the DTCs and perform a test drive under conditions similar to those noted in the freeze frame data to confirm the repair has resolved the issue and the code does not return.