Experiencing issues with your 2007 Subaru Forester OBD2 port can be frustrating, especially when you need to diagnose a Check Engine Light (CEL) or other car troubles. If your scan tool isn’t powering up when plugged into the OBD2 port of your 2007 Subaru Forester, you’re not alone. This issue is often reported, and understanding the potential causes and solutions can save you time and money.
One common scenario, particularly with older vehicles like the 2007 Subaru Forester, involves problems arising after an aftermarket head unit installation. It’s not unusual for such installations to inadvertently affect the OBD2 port’s functionality. Let’s delve into the possible reasons behind a non-functional OBD2 port in your 2007 Subaru Forester and how to address them.
Diagnosing the OBD2 Port Power Issue
The OBD2 port requires power and ground to operate correctly. A mechanic’s check revealing “12v and one ground but one was missing” points directly to a power or ground circuit problem. In many cases, a missing ground is the culprit.
Common Causes for a Missing Ground or Power Issue:
- Blown Fuse: The most common and easiest issue to check. The OBD2 port shares fuses with other systems, so a blown fuse can disable the port. Refer to your 2007 Subaru Forester owner’s manual for the fuse box diagram and identify the relevant fuse for the OBD2 port and potentially the cigarette lighter/accessory socket, as they are often related.
- Wiring Issues from Aftermarket Head Unit Installation: As mentioned earlier, aftermarket head unit installations can sometimes disrupt the vehicle’s wiring. Incorrect wiring or loose connections during the installation process can affect the ground or power supply to the OBD2 port.
- Broken or Damaged Wiring: Over time, wiring can become damaged due to wear and tear, rodent damage, or accidental cuts. Inspect the wiring around the OBD2 port and tracing back towards the fuse box and head unit area if an aftermarket unit is installed.
- Faulty OBD2 Port Connector: While less common, the OBD2 port connector itself can become faulty or corroded, leading to connection problems.
The Mystery of the Small Black Plug
The discovery of a “small black plug/socket feeding out of the same Molex as the 12v power to the cigarette/aux socket” is intriguing. This plug, especially if broken, could be related to the issues you’re experiencing. Given that shorting it affected the dash lights, it’s highly likely this plug is related to the vehicle’s illumination or dimming system, possibly for the original factory head unit.
Potential Impact of the Black Plug:
- Unrelated to OBD2 Port (Potentially): While electrical issues can be interconnected, it’s possible this black plug is for a feature unrelated to the OBD2 port’s power or ground.
- Indirect Electrical Issue: If this plug’s wiring is damaged or causing a short, it could be impacting other electrical circuits, potentially including the OBD2 port indirectly by affecting a shared ground or power source.
Troubleshooting Steps for Your 2007 Subaru Forester OBD2 Port
- Check Fuses: Locate the fuse box (usually under the dashboard or in the engine bay) and check the fuses related to the OBD2 port and accessory power socket. Replace any blown fuses with the correct amperage rating.
- Inspect Aftermarket Head Unit Wiring: If an aftermarket head unit is installed, carefully examine the wiring behind it. Look for any loose connections, cut wires, or improper splices. Consult the wiring diagrams for your 2007 Subaru Forester and the aftermarket head unit if available. Reconnect any loose wires and ensure proper insulation.
- Examine OBD2 Port Wiring: Visually inspect the wiring harness leading to the OBD2 port for any signs of damage, breaks, or corrosion.
- Test for Ground and Power: Use a multimeter to test for both 12V power and ground at the OBD2 port connector. Pin 16 should have 12V+, and pins 4 and 5 should be ground. If either is missing, trace the wiring back to identify the break.
- Investigate the Black Plug: Further investigate the broken black plug. If possible, try to identify its original location and function. Even if seemingly unrelated to the OBD2 port directly, repairing this broken connection could resolve broader electrical issues that might be indirectly affecting the OBD2 system.
- Consult a Wiring Diagram: Obtain a wiring diagram for your 2007 Subaru Forester. This diagram will be invaluable in tracing the circuits for the OBD2 port, accessory socket, and any related systems.
Lost Ground and Head Unit Swaps: Is There a Connection?
The mechanic’s comment about “lost ground after a head unit swap” being common suggests a known issue. During an aftermarket head unit installation, ground wires might be inadvertently disconnected, improperly grounded, or connected to the wrong points. This is especially true if the installer wasn’t familiar with the specific wiring of a 2007 Subaru Forester.
Moving Forward
Troubleshooting electrical issues can be methodical. Start with the simplest checks like fuses and visually inspecting wiring. If the problem persists, systematic testing with a multimeter and a wiring diagram is crucial. Given the potential complexity and the aftermarket head unit involvement, seeking assistance from a qualified mechanic experienced with Subaru vehicles and car audio installations might be the most efficient path to resolving your 2007 Subaru Forester OBD2 port issue.
Remember to prioritize safety when working with automotive electrical systems. Disconnect the negative battery terminal before undertaking any wiring inspections or repairs to prevent accidental shorts or electrical damage.
