The P1871 diagnostic trouble code (DTC) indicates that the vehicle's powertrain control module (PCM) has detected an issue with the calculated gear ratio within the transmission. This code is generally associated with automatic transmissions and suggests a discrepancy between the expected gear ratio and the actual gear ratio based on input and output shaft speeds. Resolving this issue is crucial for maintaining optimal vehicle performance, fuel efficiency, and transmission longevity.
| Component/Symptom | Description | Possible Causes |
|---|---|---|
| Gear Ratio Calculation | The PCM uses input shaft speed (turbine speed) and output shaft speed to calculate the current gear ratio. | Incorrect sensor readings, PCM malfunction. |
| Input Shaft Speed Sensor (ISS) | Measures the speed of the transmission's input shaft. | Faulty sensor, wiring issues (shorts, opens, corrosion), damaged reluctor ring. |
| Output Shaft Speed Sensor (OSS) | Measures the speed of the transmission's output shaft. | Faulty sensor, wiring issues (shorts, opens, corrosion), damaged reluctor ring. |
| Transmission Slippage | Excessive slippage within the transmission can lead to an incorrect gear ratio calculation. | Worn clutches, low transmission fluid, contaminated transmission fluid, internal transmission damage. |
| Transmission Fluid Level & Condition | Low fluid levels or degraded fluid can affect transmission performance and sensor readings. | Fluid leak, infrequent fluid changes, incorrect fluid type. |
| Valve Body Issues | Malfunctioning solenoids or valves within the valve body can prevent proper gear selection and hydraulic pressure. | Stuck solenoids, damaged valves, internal leaks within the valve body. |
| PCM (Powertrain Control Module) | The computer that controls the engine and transmission. | Rare, but possible PCM failure or corrupted software. |
| Wiring and Connectors | Damaged or corroded wiring and connectors can interrupt sensor signals. | Physical damage, corrosion, loose connections. |
| Shift Solenoids | Electromagnetically controlled valves that control fluid flow to engage different gears. | Faulty solenoids, wiring issues, mechanical blockage. |
| Mechanical Transmission Failure | Internal damage to gears, bearings, or other mechanical components. | Wear and tear, lack of maintenance, severe driving conditions. |
| Symptoms | Check engine light, harsh shifting, slipping transmission, failure to shift, reduced fuel economy, vehicle may be stuck in a particular gear. | Varies depending on the underlying cause. |
| Diagnostic Tools | Scan tool (OBD-II scanner), multimeter, wiring diagrams. | Necessary for accurate diagnosis and troubleshooting. |
| Related Codes | P0720 (Output Speed Sensor Circuit Malfunction), P0715 (Input/Turbine Speed Sensor Circuit Malfunction), P0730 (Incorrect Gear Ratio), other transmission-related codes. | Often accompany P1871 and can provide further diagnostic clues. |
| Common Misdiagnoses | Replacing sensors without proper testing, assuming the PCM is faulty, ignoring fluid level and condition. | Thorough testing and inspection are crucial before replacing any parts. |
| Severity | Moderate to Severe. Can lead to further transmission damage if not addressed. | Addressing the issue promptly can prevent more costly repairs. |
| Possible Solutions | Replacing faulty speed sensors, repairing wiring issues, replacing or flushing transmission fluid, repairing or replacing the valve body, rebuilding or replacing the transmission, PCM reprogramming or replacement. | Depends on the root cause of the problem. |
Detailed Explanations
Gear Ratio Calculation: The PCM constantly monitors the input and output shaft speeds of the transmission. It uses these readings to calculate the actual gear ratio that the transmission is currently operating in. If the calculated ratio doesn't match the expected ratio for the selected gear, the P1871 code is triggered. This discrepancy can be caused by sensor failures, transmission slippage, or PCM issues.
Input Shaft Speed Sensor (ISS): The ISS, also known as the turbine speed sensor, measures the speed at which the transmission's input shaft is rotating. This shaft is connected to the engine through the torque converter. The PCM uses this information to determine the engine's torque input into the transmission. A faulty ISS will provide inaccurate data to the PCM, leading to incorrect gear ratio calculations.
Output Shaft Speed Sensor (OSS): The OSS measures the rotational speed of the transmission's output shaft, which is connected to the driveshaft and ultimately the wheels. The PCM uses this information to determine the vehicle's speed and to calculate the gear ratio. A faulty OSS will provide inaccurate data to the PCM, leading to incorrect gear ratio calculations.
Transmission Slippage: Internal slippage within the transmission, often caused by worn clutches or bands, can cause the output shaft to rotate at a different speed than expected for a given input shaft speed. This discrepancy is detected by the PCM, resulting in the P1871 code. Slippage can be gradual or sudden, and it can worsen over time if left unaddressed.
Transmission Fluid Level & Condition: Low transmission fluid levels can lead to reduced hydraulic pressure, which is essential for proper clutch engagement and gear selection. Contaminated or degraded fluid can also affect the transmission's performance and sensor readings. Both of these issues can contribute to the P1871 code. Regularly checking and maintaining the transmission fluid is crucial.
Valve Body Issues: The valve body is a complex hydraulic control unit that directs transmission fluid to the appropriate clutches and servos to engage different gears. Malfunctioning solenoids or valves within the valve body can prevent proper gear selection and hydraulic pressure, leading to incorrect gear ratios and the P1871 code.
PCM (Powertrain Control Module): While less common, a faulty PCM can also trigger the P1871 code. The PCM is responsible for processing sensor data and controlling the transmission's operation. If the PCM is malfunctioning or its software is corrupted, it may incorrectly calculate the gear ratio or misinterpret sensor readings.
Wiring and Connectors: The wiring and connectors that connect the speed sensors to the PCM are essential for transmitting accurate data. Damaged, corroded, or loose wiring and connectors can interrupt sensor signals, leading to inaccurate readings and the P1871 code. Thoroughly inspect the wiring and connectors for any signs of damage or corrosion.
Shift Solenoids: Shift solenoids are electrically controlled valves within the transmission that regulate the flow of hydraulic fluid to engage different gears. A faulty shift solenoid can cause the transmission to get stuck in a particular gear or to shift erratically, leading to an incorrect gear ratio and the P1871 code.
Mechanical Transmission Failure: Internal damage to the transmission's gears, bearings, or other mechanical components can also cause the P1871 code. This type of failure can result in slippage, erratic shifting, and other transmission problems. In severe cases, the transmission may need to be rebuilt or replaced.
Symptoms: The symptoms associated with the P1871 code can vary depending on the underlying cause. Common symptoms include a check engine light, harsh shifting, slipping transmission, failure to shift, reduced fuel economy, and the vehicle may be stuck in a particular gear. The severity of the symptoms can range from mild to severe.
Diagnostic Tools: A scan tool (OBD-II scanner) is essential for reading the P1871 code and any other related codes. A multimeter can be used to test the speed sensors and wiring for continuity and voltage. Wiring diagrams are helpful for tracing the wiring circuits and identifying potential problems.
Related Codes: The P1871 code is often accompanied by other transmission-related codes, such as P0720 (Output Speed Sensor Circuit Malfunction), P0715 (Input/Turbine Speed Sensor Circuit Malfunction), and P0730 (Incorrect Gear Ratio). These related codes can provide further diagnostic clues and help pinpoint the root cause of the problem.
Common Misdiagnoses: One of the most common misdiagnoses is replacing the speed sensors without proper testing. It's crucial to test the sensors and wiring before replacing any parts. Another common mistake is assuming the PCM is faulty without ruling out other possible causes. Ignoring the transmission fluid level and condition is also a common oversight.
Severity: The severity of the P1871 code is generally considered moderate to severe. If left unaddressed, the underlying issue can lead to further transmission damage and potentially require more costly repairs. Addressing the problem promptly can prevent more serious issues.
Possible Solutions: The solution to the P1871 code depends on the root cause of the problem. Possible solutions include replacing faulty speed sensors, repairing wiring issues, replacing or flushing transmission fluid, repairing or replacing the valve body, rebuilding or replacing the transmission, and PCM reprogramming or replacement. A proper diagnosis is essential for determining the appropriate course of action.
Frequently Asked Questions
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What does the P1871 code mean? The P1871 code indicates an "Undefined Gear Ratio" in the transmission, meaning the PCM detects a discrepancy between the expected and actual gear ratio.
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What are the common causes of the P1871 code? Common causes include faulty speed sensors, low or contaminated transmission fluid, valve body issues, and internal transmission problems.
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Can I drive my car with the P1871 code? It's not recommended to drive extensively with the P1871 code, as it can lead to further transmission damage. Seek professional diagnosis and repair as soon as possible.
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How do I diagnose the P1871 code? Diagnosis involves using a scan tool to read the code, testing speed sensors and wiring, checking transmission fluid level and condition, and potentially performing more advanced transmission diagnostics.
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How do I fix the P1871 code? The fix depends on the underlying cause and can range from replacing a speed sensor to rebuilding or replacing the transmission.
Conclusion
The P1871 diagnostic trouble code signifies an issue with the gear ratio calculation within the transmission, typically an automatic one. Addressing the underlying cause promptly is crucial to prevent further transmission damage and maintain optimal vehicle performance. A thorough diagnosis, using appropriate tools and techniques, is essential for identifying the root cause and implementing the correct repair strategy.