P1900

What Does Code P1900 Mean?

DTC P1900 signifies an intermittent malfunction within the Output Shaft Speed (OSS) sensor circuit. The Output Shaft Speed sensor is a critical inductive or Hall-effect sensor typically located on the transmission’s output shaft, responsible for reporting the rotational speed of the transmission’s output to the Powertrain Control Module (PCM), also known as the Engine Control Module (ECM) or Transmission Control Module (TCM). The PCM utilizes this input for various functions including accurate gear ratio calculation, torque converter clutch engagement strategies, transmission shift scheduling, and often as a primary input for overall vehicle speed determination (which may then be relayed to other modules like ABS/ESP). When the PCM detects an inconsistent, implausible, or intermittently absent signal from the OSS sensor circuit, particularly when compared to other speed inputs (such as the Input Shaft Speed (ISS) sensor or wheel speed sensors via the CAN bus), it interprets this as an intermittent malfunction and sets code P1900. The “intermittent” aspect implies that the fault is not constant but occurs sporadically, making diagnosis more challenging than a hard failure.

Common Symptoms

• Malfunction Indicator Lamp (MIL) Illumination: The “Check Engine” light will be illuminated on the dashboard.
• Erratic Speedometer or Odometer: The speedometer may jump, drop to zero, or read incorrectly while driving.
• Harsh or Delayed Shifting: The transmission may exhibit hard shifts, delayed gear engagements, or reluctance to shift due to incorrect speed data.
• Transmission Limp Mode: The transmission may enter a protective “limp home” mode, often locking into a single gear (e.g., 3rd gear) to prevent further damage.
• Loss of Cruise Control Functionality: Since cruise control relies on accurate vehicle speed, it may become inoperable.
• Reduced Fuel Economy: Improper shift points or torque converter lock-up can lead to inefficient operation.
• Inconsistent Torque Converter Clutch (TCC) Operation: The TCC may fail to engage, engage erratically, or disengage prematurely.

What Causes the Code P1900?

• Faulty Output Shaft Speed (OSS) Sensor: Internal electrical failure, debris accumulation on the sensor tip, or a damaged tone ring/reluctor wheel within the transmission.
• Wiring Harness Issues: Chafed, corroded, open, or shorted wiring in the OSS sensor circuit, often near the transmission housing where it’s exposed to heat, vibration, and road debris.
• Poor Electrical Connections: Loose, corroded, or damaged terminals at the OSS sensor connector or at the PCM/TCM connector.
• Transmission Fluid Contamination: Metal debris in the transmission fluid can adhere to the magnetic tip of an inductive OSS sensor, interfering with its signal.
• PCM/TCM Internal Fault: While less common, an internal fault within the control module itself could cause it to misinterpret or fail to receive the OSS signal intermittently.

How to Diagnose and Troubleshoot

Diagnosing an intermittent fault requires patience and systematic testing. Begin by obtaining vehicle-specific wiring diagrams and component location data for the OSS sensor.

1. Perform Initial Scan & Freeze Frame Data Analysis: Connect an OBD-II scan tool and confirm P1900. Check for any accompanying transmission-related DTCs. Analyze freeze frame data to note engine speed, vehicle speed, and transmission parameters at the time the code was set. This can provide clues about specific operating conditions during the fault.
2. Visual Inspection of the OSS Circuit:
   • Locate the OSS sensor on the transmission. Visually inspect the sensor itself for any physical damage, cracks, or signs of impact.
   • Inspect the wiring harness leading to the OSS sensor for signs of chafing, fraying, cuts, or exposure to excessive heat. Pay close attention to areas where the harness passes near hot exhaust components or sharp edges.
   • Examine the OSS sensor connector and the corresponding PCM/TCM connector for bent, corroded, or pushed-out pins. Ensure connections are secure.
3. Monitor Live Data: With the scan tool connected, monitor the OSS sensor live data parameter (often labeled “Output Speed Sensor,” “Output Shaft Speed,” or “Transmission Output Speed”) while driving the vehicle under various conditions that mimic the freeze frame data. Look for sudden drops, erratic readings, or periods where the signal completely disappears. Compare the OSS reading to the Vehicle Speed Sensor (VSS) reading, if available, and ensure they are consistent after accounting for gear ratios. A slight discrepancy is normal; erratic jumps or zero readings when moving are indicative of a fault.
4. Perform a Wiggle Test: While monitoring live data, gently wiggle, twist, and pull on sections of the OSS wiring harness, especially at the sensor and PCM/TCM connectors. If the live data signal fluctuates, drops out, or becomes erratic during this test, it points to a wiring or connection issue in that specific area.
5. Electrical Circuit Testing (Key Off, Connectors Disconnected):
   • Continuity Check: Disconnect the OSS sensor and the PCM/TCM connectors. Using a digital multimeter (DMM), check for continuity between each terminal of the OSS sensor connector and its corresponding terminal at the PCM/TCM connector. Resistance should be very low (typically less than 1 ohm). An open circuit indicates a broken wire.
   • Short to Ground Check: With the DMM set to resistance (ohms), check for continuity between each terminal of the OSS sensor connector and a known good chassis ground. There should be no continuity (infinite resistance). If continuity exists, it indicates a short to ground.
   • Short to Power Check: Set the DMM to voltage and check for voltage between each terminal of the OSS sensor connector and ground with the ignition key in the ON position. There should be no voltage present. If voltage is detected, it indicates a short to power.
   • Sensor Resistance Check: If manufacturer specifications are available, check the internal resistance of the OSS sensor itself by probing its terminals. An out-of-spec reading indicates an internal sensor fault.
6. Voltage and Ground Reference Check (Key On, Sensor Connected): With the OSS sensor connected and the ignition ON, check for the presence of the specified reference voltage (typically 5V or 12V, depending on the system) and a good ground at the sensor connector using a DMM. No voltage or a poor ground indicates an issue with the PCM or the power/ground supply circuit.

Recommended Repairs and Solutions

Based on the thorough diagnostic process, the following repairs are commonly indicated:

• Replace the Output Shaft Speed (OSS) Sensor: If diagnostic tests (live data analysis, resistance checks) point to an internal sensor failure or if the sensor is physically damaged, replacement is the primary solution. Ensure to use an OEM-quality replacement sensor for optimal performance and longevity.
• Repair or Replace Wiring Harness: If the wiggle test or electrical circuit testing identified opens, shorts, or high resistance in the wiring, repair the damaged section of the harness. Use appropriate gauge wire, solder connections, and protect them with heat shrink tubing. If the damage is extensive, a section of the harness may need to be replaced.
• Clean and Secure Electrical Connectors: If corrosion or loose terminals were found, clean the connector terminals using an electrical contact cleaner and a small brush. Ensure all pins are properly seated and make firm contact. Apply dielectric grease to prevent future corrosion.
• Check Transmission Fluid Condition: If the OSS sensor is an inductive type and metal debris in the fluid is suspected, a transmission fluid and filter change may be beneficial. This should be done only after addressing the primary electrical fault.
• PCM/TCM Replacement/Reprogramming: Only consider PCM/TCM replacement as a last resort, after ruling out all other possibilities. This is rare for an intermittent OSS circuit fault. If a module replacement is necessary, it will typically require programming to the vehicle.

After performing any repair, clear the DTCs with the scan tool and perform an extended test drive under varying conditions to ensure the fault does not return. Monitoring the OSS live data during this test drive is highly recommended to confirm the signal is stable and accurate.