Experiencing limp mode in your vehicle can be frustrating and concerning, especially when accompanied by an OBDII error code. If you’re a Saab 9-3 owner and have encountered the P0638 code, you’re likely dealing with issues related to your Throttle Actuator Control (TAC) system. This article delves into a real-world scenario of troubleshooting this very problem on a 2005 Saab 9-3 Convertible 2.0t, providing insights and potential solutions for this common automotive headache.
Initial Symptoms and Diagnostic Codes
The owner of a 2005 Saab 9-3 Convertible began experiencing intermittent limp mode and even stalling. Using a basic OBDII reader, initial diagnostic trouble codes (DTCs) pointed towards throttle body related problems. The codes retrieved were:
- P2135: Throttle/Pedal Position Sensor/Switch “A” / “B” Voltage Correlation
- P1523: Throttle Actuator Control System – Forced Limited Power
- P0638: Throttle Actuator Control Range/Performance (Bank 1)
These codes collectively suggest a problem within the throttle position sensor circuit or the throttle actuator itself, impacting the engine’s ability to control airflow and leading to limp mode as a safety measure. The owner initially observed discrepancies in throttle position sensor readings using an OBDII app, further reinforcing suspicions about the throttle body.
First Attempts at Repair and Unexpected Fuel Pump Code
The first steps taken were typical DIY approaches to address throttle body issues. Cleaning the throttle body and clearing the error codes offered a temporary respite, but the problem soon returned. Interestingly, alongside the recurring throttle-related codes (P2135, P1523, P0638), a new code emerged:
- P0628: Fuel Pump Control Circuit Low
This P0628 code, related to the fuel pump, was an unexpected addition and suggested a potentially broader electrical or sensor issue beyond just the throttle body. While the fuel pump code did not reappear, it highlighted the complexity of diagnosing intermittent automotive problems.
To further troubleshoot, the Mass Air Flow (MAF) sensor and the H42-2 connector, known to cause issues in Saab vehicles, were cleaned. A “pedal reset trick,” intended to recalibrate throttle position sensors, was also attempted. These actions provided temporary relief, suggesting that connection issues or sensor contamination might be contributing factors, but not the root cause.
Escalating Issues and Throttle Body Replacement
Despite these initial efforts, the limp mode and the original error codes (P2135, P1523, P0638) returned. Suspecting a faulty throttle body, a replacement was installed. This seemed to resolve the issue for a longer period – two months – before the dreaded limp mode reappeared, accompanied by a more serious symptom: stalling at intersections.
This escalation, including stalling, underscored the urgency and potential safety implications of the problem. Further investigation revealed that the new throttle body, like the original, produced a rattling sound when shaken. This raised questions about whether the rattling was normal for these units or indicative of a widespread defect. Despite the rattle, a second replacement throttle body was installed, along with further cleaning of the H42-2 connector, the Manifold Absolute Pressure (MAP) sensor (located in the air intake tube), and even the ECU connectors. Visual inspection for wiring breaks proved inconclusive.
Persistent Problem and Seeking Professional Help
Unfortunately, even with a second new throttle body and extensive cleaning of related components and connectors, the limp mode and the P0638, P2135, and P1523 codes persisted. This indicated that the issue was likely not a simple throttle body malfunction but potentially a more complex electrical problem, possibly involving wiring shorts or a failing ECU.
Faced with the persistent problem and the potential for a deeper electrical or computer issue, the Saab 9-3 owner decided to seek professional help from a Saab specialist auto shop. This decision highlights the point where DIY troubleshooting reaches its limits, and expert diagnostics become necessary.
Potential Causes and Diagnostic Steps for P0638
Based on this real-world example and general automotive knowledge, OBDII error code P0638, “Throttle Actuator Control Range/Performance,” can stem from several potential issues:
- Faulty Throttle Body/Actuator: While throttle body replacement was attempted in this case, manufacturing defects or subtle internal failures can still occur. Testing the throttle body with specialized equipment might be needed.
- Wiring and Connector Issues: Problems in the wiring harness leading to the throttle body, including shorts, open circuits, or corrosion in connectors (like the H42-2 in Saabs), can disrupt signals and trigger P0638.
- Throttle Position Sensor (TPS) Problems: Although P2135 also points to TPS issues, a faulty TPS within the throttle body or its circuit can directly cause P0638.
- ECU/PCM Malfunction: In rarer cases, the Engine Control Unit (ECU) or Powertrain Control Module (PCM) itself may be faulty, misinterpreting signals or failing to properly control the throttle actuator.
- MAF or MAP Sensor Issues: While less direct, incorrect readings from the MAF or MAP sensor can indirectly affect throttle control and potentially contribute to P0638.
- Mechanical Throttle Plate Binding: Physical obstruction or binding of the throttle plate within the throttle body can impede its movement and trigger the error.
Diagnostic Steps to Consider:
- OBDII Code Scan and Freeze Frame Data: Reconfirm P0638 and any related codes. Analyze freeze frame data for clues about conditions when the code set.
- Throttle Body Inspection and Testing: Visually inspect the throttle body and wiring. Use a multimeter to check throttle actuator motor resistance, TPS voltages, and circuit continuity.
- Wiring Harness and Connector Checks: Carefully inspect wiring harnesses and connectors (especially H42-2 in Saabs) for damage, corrosion, or loose pins. Test wiring continuity between the ECU, throttle body, and pedal position sensor.
- Sensor Data Monitoring: Use a scan tool to monitor live data from the TPS, MAF, MAP, and other relevant sensors to identify any erratic or out-of-range readings.
- ECU/PCM Testing (Advanced): If other tests are inconclusive, ECU/PCM diagnostics might be necessary, potentially requiring specialized tools and expertise.
Conclusion
Troubleshooting OBDII error code P0638 can be complex, as demonstrated by this Saab 9-3 case. While initial steps like cleaning connectors and even throttle body replacement may offer temporary solutions, persistent issues often indicate deeper electrical or component failures. Seeking professional diagnostic help from experienced technicians is crucial for accurately pinpointing the root cause and implementing effective repairs to resolve limp mode and error code P0638, ensuring vehicle safety and performance. Stay tuned for updates on the final diagnosis and resolution of this Saab 9-3 throttle control mystery.
