Introduction:
The diagnostic trouble code (DTC) B1991, "Pedal Forward / Rearward Potentiometer Feedback Circuit Short to Battery," indicates an electrical fault within the system responsible for monitoring the accelerator pedal position. This issue can significantly impact vehicle performance, potentially leading to unintended acceleration, sluggish response, or even disabling the vehicle's ability to move. Understanding the causes, symptoms, and troubleshooting steps for this code is crucial for accurate diagnosis and repair.
Comprehensive Table: B1991 Pedal Forward / Rearward Potentiometer Feedback Circuit Short to Battery
| Topic | Description | Possible Solutions |
|---|---|---|
| Code Definition | B1991 indicates that the control module (typically the PCM or ECM) has detected an excessively high voltage on the pedal position sensor's feedback circuit. This suggests the circuit is shorted to a voltage source (battery voltage). The "Forward/Rearward" designation refers to how some vehicles use separate potentiometers for forward and rearward pedal travel. | Verify wiring integrity, replace damaged wiring harness, repair short to voltage, inspect and replace faulty pedal position sensor, reprogram or replace the control module (PCM/ECM) as a last resort. Thoroughly inspect connectors for corrosion or damage. |
| Common Symptoms | Erratic or unintended acceleration, sluggish throttle response, engine stalling, idle problems, inability to start the vehicle, throttle position sensor (TPS) warning light illumination, transmission shifting issues (if the TPS signal is used for transmission control), limp mode activation. The severity varies depending on the vehicle and the extent of the short. | Address the root cause of the short circuit. Resolving the electrical fault will typically eliminate the symptoms. Clearing the DTC after repair is essential. |
| Potential Causes | Damaged wiring harness (chafing, rodent damage, physical damage), corroded or loose connectors, faulty pedal position sensor (PPS), internal short within the PPS, short circuit to battery voltage within the wiring harness, failed PCM/ECM (rare, but possible). Moisture intrusion into the wiring or sensor can also cause issues. | Isolate the short circuit and repair. Replace damaged components (wiring, connectors, sensor). If the PCM/ECM is suspected, professional diagnosis and programming are required. Address any underlying causes of the damage (e.g., rodent infestation). |
| Diagnostic Tools | Multimeter, scan tool with live data capabilities, wiring diagrams specific to the vehicle, oscilloscope (optional, for advanced diagnostics), circuit tester, connector test kit. Understanding how to use these tools is crucial for accurate diagnosis. | Use the appropriate tools to test the circuit and components. Refer to the vehicle's service manual for specific testing procedures. |
| Affected Components | Pedal position sensor (PPS)/Accelerator Pedal Sensor (APS), wiring harness connecting the PPS to the PCM/ECM, PCM/ECM itself, connectors within the circuit. The throttle body (if electronically controlled) can also be indirectly affected. | Inspect and test each of these components. Replace any faulty components. Ensure proper connections and wiring integrity. |
| Wiring Diagram Relevance | Essential for tracing the circuit and identifying potential short locations. Shows the wiring routes, connector locations, and pin assignments. Provides voltage and resistance specifications for testing. Without a wiring diagram, troubleshooting becomes significantly more difficult and prone to error. | Obtain the correct wiring diagram for the specific vehicle year, make, and model. Use the diagram to guide your testing and repair efforts. |
| Voltage Testing | Use a multimeter to measure the voltage at the PPS connector. A short to battery will typically show a voltage close to battery voltage (12V+) on the signal wire when it should be significantly lower. Check the ground connection for proper continuity. | Compare voltage readings to the specifications in the wiring diagram. Identify and isolate the short circuit. |
| Resistance Testing | Measure the resistance of the PPS signal wire to ground and to battery positive. A low resistance to battery positive indicates a short to battery. Measure the resistance of the PPS itself according to the manufacturer's specifications. | Compare resistance readings to the specifications in the wiring diagram. Identify and isolate the short circuit. |
| Scan Tool Data | Monitor the TPS/APS PID (Parameter Identification Data) using a scan tool. Look for abnormally high voltage readings, erratic fluctuations, or a fixed voltage value. Compare the TPS/APS reading to the actual pedal position. Check for any related DTCs. | Use the scan tool data to confirm the presence of a short to battery and to identify any other related issues. |
| Safety Precautions | Disconnect the battery negative terminal before working on the electrical system. Use caution when working with electrical components. Avoid touching exposed wires. Wear appropriate safety gear (gloves, eye protection). Consult the vehicle's service manual for specific safety recommendations. | Follow all safety precautions to prevent electrical shock and damage to the vehicle. |
| Limp Mode Implications | If the PCM/ECM detects a severe fault in the TPS/APS circuit, it may activate limp mode to protect the engine and transmission. Limp mode typically restricts engine power and speed. This is a safety feature designed to prevent further damage. | Address the underlying cause of the B1991 code to resolve the limp mode condition. |
| Connector Inspection | Thoroughly inspect all connectors in the PPS circuit for corrosion, bent or broken pins, and loose connections. Clean corroded connectors with electrical contact cleaner. Repair or replace damaged connectors as needed. Ensure proper locking mechanisms are engaged. | Clean, repair, or replace any damaged connectors. Ensure proper connections. |
| Rodent Damage Assessment | Rodents often chew on wiring harnesses, causing shorts and open circuits. Look for signs of rodent activity, such as chewed wires, droppings, or nesting materials. Repair any rodent damage to the wiring harness. Take steps to prevent future rodent infestations. | Repair or replace any wiring damaged by rodents. Implement rodent control measures. |
| Aftermarket Accessories | Aftermarket accessories (e.g., remote starters, performance chips) can sometimes interfere with the PPS circuit. Disconnect any aftermarket accessories to see if the B1991 code clears. If the code clears, the aftermarket accessory is likely the cause of the problem. | Disconnect or remove any aftermarket accessories that are interfering with the PPS circuit. |
Detailed Explanations:
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Code Definition: The B1991 DTC signifies that the vehicle's computer (PCM/ECM) has detected a higher-than-expected voltage on the feedback circuit of the pedal position sensor. This suggests a short circuit where the sensor wiring is making unintended contact with a source of power, typically the vehicle's battery voltage. The "Forward/Rearward" aspect indicates some vehicles utilize separate potentiometers to measure pedal movement in both directions.
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Common Symptoms: The symptoms of a B1991 code can range from subtle to severe. Common indicators include erratic acceleration, delayed throttle response, engine stalling, and idle inconsistencies. In some cases, the vehicle may enter "limp mode," restricting engine power. The check engine light will almost always be illuminated.
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Potential Causes: The most frequent causes of a B1991 code are related to wiring issues. These include damaged wiring harnesses (chafing against the body, rodent damage), corroded or loose connectors, and internal shorts within the pedal position sensor itself. A less common, but possible, cause is a failure within the PCM/ECM.
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Diagnostic Tools: Diagnosing a B1991 code requires a few key tools. A multimeter is essential for voltage and resistance testing. A scan tool allows you to read the code and monitor live data from the pedal position sensor. Wiring diagrams specific to the vehicle are crucial for tracing the circuit. An oscilloscope can be helpful for advanced diagnostics, allowing you to visualize the signal from the sensor.
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Affected Components: The primary components involved are the pedal position sensor (PPS), the wiring harness connecting the PPS to the PCM/ECM, and the PCM/ECM itself. The throttle body (if electronically controlled) can also be indirectly affected, as it relies on the signal from the PPS to regulate engine speed.
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Wiring Diagram Relevance: A wiring diagram is indispensable for diagnosing this issue. It provides a detailed map of the circuit, showing the wire colors, connector locations, and pin assignments. This allows you to trace the circuit, identify potential short locations, and verify the integrity of the wiring.
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Voltage Testing: Voltage testing is a key step in diagnosing a B1991 code. Using a multimeter, you can measure the voltage at the PPS connector. A short to battery will typically show a voltage close to battery voltage (12V+) on the signal wire when it should be significantly lower. It's also important to check the ground connection for proper continuity.
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Resistance Testing: Resistance testing complements voltage testing. By measuring the resistance of the PPS signal wire to ground and to battery positive, you can confirm the presence of a short to battery. A low resistance to battery positive strongly indicates a short. You should also measure the resistance of the PPS itself according to the manufacturer's specifications.
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Scan Tool Data: A scan tool provides valuable information about the pedal position sensor. By monitoring the TPS/APS PID (Parameter Identification Data), you can observe the sensor's voltage readings. Abnormally high voltage readings, erratic fluctuations, or a fixed voltage value are all signs of a problem.
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Safety Precautions: When working on the electrical system, it's crucial to prioritize safety. Always disconnect the battery negative terminal before starting any work. Use caution when handling electrical components and avoid touching exposed wires. Wearing appropriate safety gear (gloves, eye protection) is also recommended.
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Limp Mode Implications: If the PCM/ECM detects a severe fault in the TPS/APS circuit, it may activate limp mode. Limp mode restricts engine power and speed to prevent further damage. Resolving the underlying cause of the B1991 code is necessary to restore normal vehicle operation.
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Connector Inspection: Corroded or damaged connectors are a common cause of electrical problems. Thoroughly inspect all connectors in the PPS circuit for corrosion, bent or broken pins, and loose connections. Clean corroded connectors with electrical contact cleaner and repair or replace damaged connectors as needed.
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Rodent Damage Assessment: Rodents can cause significant damage to wiring harnesses. Look for signs of rodent activity, such as chewed wires, droppings, or nesting materials. Repair any rodent damage to the wiring harness and take steps to prevent future infestations.
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Aftermarket Accessories: Aftermarket accessories can sometimes interfere with the PPS circuit. Disconnect any aftermarket accessories to see if the B1991 code clears. If the code clears, the aftermarket accessory is likely the cause of the problem.
Frequently Asked Questions:
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What does the B1991 code mean? It means the vehicle's computer has detected a short to battery on the pedal position sensor feedback circuit.
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Can I drive my car with a B1991 code? It is not recommended. The code can cause unpredictable acceleration or stalling, making driving unsafe.
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Is the pedal position sensor expensive to replace? The cost varies depending on the vehicle, but it's typically a moderately priced part.
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Can a bad PCM cause a B1991 code? Yes, but it's less common than wiring or sensor issues. The PCM should be the last thing to replace.
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How do I fix a short to battery? Carefully inspect the wiring harness for damage and repair any broken or chafed wires.
Conclusion:
The B1991 DTC indicates a potentially serious electrical fault that requires careful diagnosis and repair. By systematically inspecting the wiring, connectors, and pedal position sensor, you can identify and resolve the short to battery. Always prioritize safety and consult the vehicle's service manual for specific procedures.