08 Equinox OBD2 Mystery: Decoding Diagnostic Dilemmas

It can be incredibly frustrating when your car throws you for a loop, especially when it involves the engine and diagnostic systems. You might be experiencing something similar to a recent query we received about a 2009 Chevy Malibu, which, while not an 08 Equinox, shares similar GM engineering and potential diagnostic quirks of that era. The owner reported a sudden loss of power and illuminated dash lights in their Malibu. Initially, hooking up an OBD2 scanner revealed codes U0073 and P0700. However, after some basic troubleshooting, the car seemed to return to normal, leaving the owner questioning the initial diagnostic findings and next steps. This situation highlights a common concern: what happens when your OBD2 port seems to give you confusing or incomplete information, especially in vehicles around the 2008 model year, including models like the Chevy Equinox?

Let’s delve into this scenario and explore potential causes and diagnostic approaches when you encounter a situation where your OBD2 readings might not paint the whole picture, or when intermittent issues make diagnosis challenging.

Understanding the Initial Problem: Loss of Power and Error Codes

The original poster described a sudden loss of power in their 2009 Malibu, accompanied by dash lights and a “reduced power” message. Despite pressing the gas pedal and shifting gears, the car barely moved. This immediately suggests a significant issue affecting either the engine, transmission, or both. The fact that the car later seemed to resolve itself is crucial information, pointing towards a potentially intermittent fault.

The OBD2 scanner initially pulled two codes:

• U0073 (Control Module Communication Bus Off): This code generally indicates a communication fault on the vehicle’s data bus network. Modern cars use a network to allow various control modules (like the engine control module (ECM), transmission control module (TCM), body control module (BCM), etc.) to communicate with each other. A U0073 code suggests a disruption in this communication.
• P0700 (Transmission Control System Malfunction): This is a generic transmission code indicating that the transmission control system has detected a fault and has requested the engine control module to turn on the malfunction indicator lamp (MIL), or check engine light. P0700 itself is a broad code and usually needs to be investigated further with a transmission-specific scan tool to retrieve more detailed transmission fault codes.

Troubleshooting Steps Taken and the Intermittent Nature of the Fault

The car owner took some proactive steps, including:

• Battery Disconnection: Disconnecting the battery can sometimes reset temporary faults and clear volatile memory in control modules.
• Dielectric Grease on BCM Connectors: Applying dielectric grease to electrical connectors, especially on the Body Control Module (BCM), is a good practice to prevent corrosion and ensure good electrical connections. The BCM is a central module that can affect various vehicle systems and communication networks.
• Throttle Body and MAF Sensor Cleaning: Cleaning the throttle body and Mass Air Flow (MAF) sensor is routine maintenance that can address issues related to air intake and engine performance. While less directly related to the initial symptoms described, it’s generally good preventative maintenance.

After these steps, the car seemed to operate normally, and a subsequent OBD2 scan showed no codes. This is where the diagnostic puzzle deepens. The fact that the problem disappeared after these actions doesn’t necessarily mean the underlying issue is resolved. Intermittent faults can be notoriously difficult to diagnose because they don’t always present themselves when you’re trying to troubleshoot.

Could it be the TCM (Transmission Control Module)?

The car owner raised a valid question: “can the TCM be going?” Given the P0700 code and the nature of the symptoms (loss of drive), a transmission control module issue is certainly a possibility.

Here’s why a TCM could be involved, and what to consider:

• TCM’s Role: The TCM is responsible for managing the automatic transmission’s operation, including gear shifting, torque converter lockup, and overall transmission health.
• Intermittent TCM Faults: TCMs, like any electronic module, can experience intermittent failures due to factors like temperature changes, vibrations, or internal component degradation. These intermittent issues might not always be consistently detected by OBD2 scans, especially generic scanners.
• Communication Issues and U0073: The U0073 code is also relevant here. If there are communication problems affecting the TCM’s ability to communicate with other modules (like the ECM), it could lead to transmission malfunctions and the P0700 code, as well as the U0073 communication fault code itself.

However, simply assuming a TCM replacement is necessary would be premature. Further investigation is needed.

Deeper Diagnostic Steps for Intermittent OBD2 Issues

When faced with intermittent problems and potentially misleading OBD2 readings, especially in vehicles like the ’08 Equinox or similar GM models, consider these more in-depth diagnostic steps:

1. Use a Professional-Grade Scan Tool: Generic OBD2 scanners are useful for basic code reading, but they often lack the depth to diagnose complex issues, especially within specific modules like the TCM or ABS. A professional-grade scan tool (like those used by dealerships or specialized repair shops) can:

   • Access module-specific codes: Read more detailed codes from the TCM, ABS, BCM, and other modules beyond the generic P0700. There might be more specific transmission codes stored within the TCM’s memory that a generic scanner might miss.
   • Perform live data streaming: Monitor real-time data from various sensors and modules. This can be invaluable in catching intermittent glitches or sensor readings that are out of range when the problem occurs.
   • Perform module-specific tests: Some advanced scanners can perform tests on individual modules and components to assess their functionality.

2. Check for Stored Codes (Even if “No Codes Present”): Even if a generic scan shows “no codes” after the problem seems to disappear, a professional scan tool might still retrieve “history codes” or “pending codes” stored within the modules’ memory. These codes can provide clues about past faults that are no longer actively triggering the check engine light but are still relevant to the intermittent issue.

3. Inspect Wiring and Connectors Thoroughly: Communication issues (U0073) and intermittent electrical problems are often caused by wiring or connector faults. Carefully inspect:

   • Wiring Harnesses: Look for any signs of damage, chafing, or corrosion on wiring harnesses, especially those related to the TCM, ECM, and BCM.
   • Connectors: Check all connectors for corrosion, loose pins, or damage. Pay particular attention to connectors related to the TCM and the vehicle’s main communication bus. Dielectric grease is helpful, but sometimes connectors themselves can degrade over time.
   • Ground Points: Ensure all ground points are clean and securely fastened. Poor grounds can cause a multitude of electrical issues.

4. Consider Environmental Factors: Intermittent problems can sometimes be triggered by environmental conditions:

   • Temperature: Does the problem occur more often in hot or cold weather? Temperature changes can affect electrical components and connections.
   • Humidity/Moisture: Moisture can exacerbate corrosion and electrical shorts.
   • Vibration: Road vibrations can sometimes trigger intermittent faults in wiring or components.

5. Fluid Levels and Condition (Transmission): While the owner noted the lack of a transmission dipstick (common in many modern vehicles), it’s still crucial to assess the transmission fluid. If possible (sometimes requiring removing a fill plug and checking fluid level), check the fluid level and condition. Low fluid or degraded fluid can contribute to transmission problems. Given the age of the vehicle (11 years), even with low mileage, considering a transmission fluid and filter change might be a proactive maintenance step.

Transmission Fluid Change: Age vs. Mileage

The question about transmission fluid change is a good one, especially for a vehicle that is 11 years old but has low mileage (30,600 miles). While mileage is a primary factor in transmission fluid change intervals, time is also a factor.

• Fluid Degradation Over Time: Transmission fluid, like any fluid, can degrade over time due to oxidation and chemical breakdown, even if the vehicle isn’t driven extensively. Additives in the fluid can lose their effectiveness.
• GM Recommendations: GM’s recommended intervals for transmission fluid changes vary by model and transmission type. However, for older vehicles or in cases of uncertain maintenance history, a fluid and filter change around the 10-year mark, regardless of mileage, can be a reasonable preventative measure.

Recommendation: For an 11-year-old Malibu (or a similar age Equinox) with original transmission fluid, even with low mileage, a transmission fluid and filter change is a highly recommended maintenance item. This can help ensure proper transmission lubrication and function and potentially prevent future issues. Consult the owner’s manual or a trusted mechanic for the specific fluid type and procedure for this vehicle, especially given the lack of a dipstick, which might require a specific fill procedure.

Conclusion: Persistence in Diagnosis is Key

Dealing with intermittent OBD2 issues, especially in vehicles of a certain age, requires a systematic and persistent approach. While the initial troubleshooting steps taken by the Malibu owner were good starting points, further investigation with more advanced tools and techniques is often necessary to pinpoint the root cause of intermittent problems. Don’t be discouraged if the initial OBD2 scan doesn’t reveal the whole story. By utilizing professional diagnostic equipment, thoroughly inspecting wiring and connections, considering environmental factors, and performing recommended maintenance like transmission fluid changes, you can effectively tackle these diagnostic challenges and keep your vehicle running reliably. And remember, if you are not comfortable performing these diagnostic steps yourself, consulting a qualified mechanic specializing in automotive diagnostics is always a wise decision.