P1606

What Does Code P1606 Mean?

The diagnostic trouble code P1606 indicates that the Engine Control Module (ECM), often interchangeably referred to as the Powertrain Control Module (PCM), has detected an electrical malfunction within the output circuit responsible for controlling its own main power relay. This relay is a critical component, serving as the primary electrical switch that supplies fused 12-volt battery voltage to the ECM itself and to numerous other essential engine management components, such as fuel injectors, ignition coils, and various sensors, when the ignition key is in the “ON” or “START” position. The ECM continuously monitors the control circuit of this relay, including its voltage feedback, resistance, and current draw. When the ECM detects a deviation from the manufacturer’s specified electrical parameters—such as an open circuit, a short to ground, a short to battery voltage, or an excessive/insufficient current draw—for a predetermined period, it will register a P1606 code and illuminate the Malfunction Indicator Lamp (MIL). This code specifically points to an issue that could compromise the ECM’s ability to reliably power itself or its dependent peripherals.

Common Symptoms

• No Start Condition: The engine may crank but fail to start, or it may not crank at all if the ECM is not receiving power.
• Intermittent Starting: The vehicle may start sometimes but not others, or require multiple attempts.
• Engine Stalling: The engine may start and then immediately stall, or it may unexpectedly stall while driving.
• Multiple Other DTCs: A P1606 can often be accompanied by a host of other engine-related DTCs due to the lack of proper power supply to various components.
• Malfunction Indicator Lamp (MIL) Illumination: The “Check Engine Light” will be illuminated on the dashboard.
• Reduced Engine Performance: Erratic idle, hesitation, or a general lack of power may be experienced if the relay is providing intermittent or insufficient power.

What Causes the Code P1606?

• Faulty ECM Main Power Relay: The relay itself can fail internally, leading to intermittent operation, sticking open, or sticking closed.
• Open or Short Circuit in the Relay Control Circuit: Damage to the wiring harness (e.g., fraying, corrosion, heat damage, rodent damage) can create an open circuit (no power or ground) or a short circuit to either ground or battery voltage within the relay’s control wires.
• Corroded or Loose Electrical Connectors: Poor electrical contact at the relay socket, the ECM connector, or any inline connectors within the relay’s circuit can lead to high resistance or intermittent connection.
• Compromised Ground Connection: A poor or corroded ground point for the ECM or the relay control circuit can disrupt proper operation.
• Blown Fuse: A fuse protecting the relay’s power supply or its control circuit may be blown, although this is often a symptom of a short elsewhere rather than the root cause.
• Internal ECM Failure: In less common scenarios, the internal driver circuit within the ECM responsible for controlling the main power relay could be faulty.

How to Diagnose and Troubleshoot

Safety Precaution: Always disconnect the negative battery terminal before performing continuity or resistance checks on electrical circuits to prevent damage to electrical components or personal injury.

1. Initial Scan and Visual Inspection:
   • Connect an OBD-II scanner to confirm P1606 as the active code and check for any related or accompanying diagnostic trouble codes.
   • Visually inspect the ECM main power relay, its electrical connector, and the surrounding wiring harness for obvious signs of damage, corrosion, looseness, or insulation breaches. Pay close attention to areas prone to heat or abrasion.
   • Locate and inspect all fuses associated with the ECM and the main power relay in both under-hood and cabin fuse boxes. Test each fuse for continuity using a digital multimeter (DMM) to ensure none are blown.
2. ECM Main Relay Functionality Test:
   • Power Supply Check: With the ignition OFF, use a DMM to check for constant battery voltage (B+) at the main power input terminal (typically terminal 30) of the relay socket. This ensures the relay is receiving constant power.
   • Control Circuit Voltage/Ground Check: Identify the relay’s coil control terminals (typically 85 and 86). One terminal usually receives switched B+ from the ignition, and the other is grounded by the ECM (or vice-versa).
     • With the ignition ON (engine OFF), check for voltage at the switched B+ side of the control coil.
     • On the ECM-controlled ground side of the coil, check for continuity to a known good chassis ground or for appropriate voltage/signal from the ECM.
   • Bench Test Relay: If accessible, remove the main relay. Apply 12V to the coil terminals (85 and 86) and listen for an audible click. Then, check for continuity across the load contacts (30 and 87) using the DMM. If no click or no continuity when energized, the relay is faulty.
3. Wiring Harness Continuity and Resistance Checks:
   • Between ECM and Relay: Disconnect the ECM connector and the main power relay from their sockets. Using the vehicle’s wiring diagram, identify the specific wires that comprise the relay’s control circuit between the ECM and the relay socket. Use the DMM to check for continuity (should be near 0 ohms) and resistance in these wires. Any resistance exceeding a few ohms indicates a high-resistance fault or an open circuit.
   • Ground Circuit Integrity: Verify the integrity of the ECM’s ground circuit by checking continuity from the ECM’s ground pins (as per the wiring diagram) to a known good chassis ground. Clean and inspect all relevant ground points.
   • Short Circuit Detection: With the ECM and relay disconnected, check for continuity between each wire of the relay’s control circuit and chassis ground (to detect a short to ground) and between each wire and a constant 12V source (to detect a short to power).
4. Voltage Drop Test:
   • If the engine starts and runs, perform a voltage drop test across the relay’s power and ground supply circuits while the engine is running or with the ignition ON. Excessive voltage drop (more than 0.2V-0.5V, depending on circuit) indicates high resistance, often due to corrosion or a loose connection.
5. ECM Terminal Inspection:
   • Carefully inspect the ECM connector terminals for bent pins, corrosion, or inadequate seating, which can disrupt circuit integrity.

Recommended Repairs and Solutions

Once the root cause of P1606 has been pinpointed through systematic diagnosis, the following repairs are typically recommended:

• Replace Faulty ECM Main Power Relay: This is the most common and often the simplest fix. Ensure the replacement relay is an Original Equipment Manufacturer (OEM) equivalent or a high-quality aftermarket part, as inexpensive relays can sometimes have short lifespans or cause further issues.
• Repair or Replace Damaged Wiring: Isolate and meticulously repair any open circuits, short circuits, or areas of high resistance in the relay’s control wiring harness. Use appropriate gauge wire, solder connections (if proficient), and seal all repairs with heat-shrink tubing to ensure long-term durability and resistance to environmental factors. For extensive damage, replacing the affected section of the wiring harness may be necessary.
• Clean and Secure Electrical Connectors: Use a specialized electrical contact cleaner to thoroughly clean any corroded or contaminated terminals at the relay socket, the ECM connector, and any inline connectors within the circuit. Ensure all connectors are fully seated and locked into place.
• Address Ground Issues: Clean and tighten any loose, corroded, or otherwise compromised ground points that supply the ECM or the relay circuit. Ensure direct metal-to-metal contact.
• ECM Replacement (Last Resort): Only consider ECM replacement after all other components, wiring, and connections have been thoroughly tested and verified to be in proper working order. An internal ECM fault in the relay driver circuit is rare but possible. ECM replacement is costly and often requires specialized programming or flashing to the vehicle’s specific VIN, so ensure absolutely no other possibilities exist before proceeding.

After completing any repairs, clear all stored DTCs using an OBD-II scanner. Perform a comprehensive test drive under various operating conditions to ensure the code does not return and that all systems are functioning correctly. Monitor live data related to ECM power supply or relay status (if available on your scanner) during the drive cycle to confirm stability.