P1874

What Does Code P1874 Mean?

DTC P1874 indicates a detected malfunction within the power supply circuit for one or more automatic transmission Hall effect sensors. The Powertrain Control Module (PCM) or Transmission Control Module (TCM), depending on vehicle architecture, monitors the voltage supplied to these crucial sensors. Hall effect sensors are typically used in automatic transmissions to monitor rotational speeds, such as input shaft speed (turbine speed), output shaft speed, and sometimes intermediate shaft speeds. These sensors require a stable reference voltage (commonly 5V or 12V, depending on manufacturer specifications) to operate correctly and generate accurate signal data. When the PCM/TCM detects that the voltage on this dedicated power circuit falls outside of the specified operating range (e.g., too low, indicating an open circuit or short to ground, or unexpectedly high, indicating a short to voltage), it registers code P1874. This directly impacts the transmission’s ability to precisely determine gear ratios, apply appropriate line pressure, and execute smooth shift strategies, as it relies heavily on accurate speed data to operate efficiently.

Common Symptoms

• Malfunction Indicator Light (MIL) illumination: The Check Engine Light will be illuminated on the dashboard.
• Transmission entering limp mode: The transmission may revert to a default gear (e.g., 2nd or 3rd) to protect itself from damage.
• Harsh or erratic shifting: Shifts may feel delayed, abrupt, or inconsistent.
• Delayed or no gear engagement: The vehicle may hesitate to move from a standstill or fail to engage a specific gear.
• Inconsistent or no speedometer/odometer reading: If the affected sensor is critical for output speed, though this is less common for a general power circuit fault.
• Reduced fuel economy: Incorrect shift patterns and TCC operation can lead to increased fuel consumption.
• Vehicle stalling: In rare cases, if the input speed signal is critical and severely compromised, leading to TCC engagement at inappropriate times.

What Causes the Code P1874?

• Open or short circuit in the sensor power supply wiring: Damage, corrosion, or fraying of the wire supplying power to the Hall effect sensor(s), leading to a loss of voltage or a short to ground/voltage.
• Faulty electrical connector(s): Loose, corroded, or damaged pins within the connectors for the transmission Hall effect sensor(s) or the PCM/TCM harness.
• Internal short circuit within the Hall effect speed sensor(s): While the code points to the power circuit, a shorted sensor itself can draw excessive current, pulling down the supply voltage and mimicking a power circuit fault.
• Blown fuse: If the power supply circuit to the transmission sensors is protected by a dedicated fuse, it could be blown, typically due to a short somewhere in the circuit.
• Faulty Powertrain Control Module (PCM) or Transmission Control Module (TCM): An internal defect within the module responsible for supplying the reference voltage or power to the sensor(s).

How to Diagnose and Troubleshoot

Diagnosing P1874 requires a systematic approach, often utilizing a professional scan tool and a digital multimeter (DMM).

1. Retrieve & Analyze DTCs: Use an OBD-II scan tool to confirm P1874 and check for any co-existing transmission-related codes. Record freeze frame data for environmental conditions under which the code was set.
2. Visual Inspection:
   • Inspect the transmission wiring harness, especially around the Hall effect speed sensors (input, output, intermediate) and where the harness routes to the PCM/TCM. Look for signs of chafing, cuts, pinches, or heat damage.
   • Examine all relevant electrical connectors for corrosion, bent pins, or loose connections. Pay close attention to the sensor connectors themselves and the main transmission harness connectors.
3. Consult Service Manual: Obtain the specific vehicle’s factory service manual or wiring diagram to identify the exact Hall effect sensors involved, their locations, and the pin-outs for their power supply and ground circuits at both the sensor and PCM/TCM connectors. Identify the expected reference voltage (e.g., 5V or 12V).
4. Power Supply Voltage Test (Sensor Connector):
   • With the ignition ON and engine OFF (KOEO), disconnect the suspected Hall effect sensor’s electrical connector.
   • Using a DMM, back-probe the power supply wire at the sensor’s harness connector. Measure voltage between this wire and a known good chassis ground. Compare to the specified reference voltage (e.g., 5V or 12V).
   • Also, measure voltage between the power supply wire and the sensor’s dedicated ground wire at the connector.
   • An absence or significant deviation from the specified voltage indicates an open circuit, short to ground, or short to voltage in the wiring harness or an issue with the PCM/TCM supply.
5. Continuity and Short Circuit Tests (Wiring Harness):
   • Disconnect both the sensor and the PCM/TCM connectors.
   • Using the DMM in continuity mode, test for continuity of the power supply wire from the PCM/TCM connector to the sensor connector. An open circuit indicates a break.
   • Test for a short to ground by checking continuity between the power supply wire and chassis ground. There should be no continuity.
   • Test for a short to voltage by checking for any voltage present on the power supply wire when it should be isolated.
6. Sensor Internal Resistance Test (If Applicable): While the code points to the power circuit, a shorted sensor can cause the power circuit to fail. If accessible, measure the internal resistance of the Hall effect sensor itself and compare it to manufacturer specifications. An open or shorted sensor could be the root cause.
7. PCM/TCM Power Output Test: If the power supply is absent at the sensor connector, but the wiring tests good, connect a breakout box or carefully back-probe the PCM/TCM connector to verify if the module is outputting the correct reference voltage on the appropriate pin. If not, the PCM/TCM may be faulty.

Recommended Repairs and Solutions

The repairs for P1874 will depend directly on the findings from the diagnostic steps:

• Repair or Replace Damaged Wiring/Connectors: This is the most common resolution. If an open circuit, short to ground, or short to voltage is found in the wiring harness, repair the damaged section using appropriate automotive-grade wire, soldering, and heat-shrink tubing. Replace corroded or damaged connectors entirely.
• Replace Faulty Hall Effect Speed Sensor(s): If the wiring tests good, but the sensor itself is found to have an internal short (causing the power circuit fault) or is otherwise defective, replace the specific sensor(s). Ensure the correct OEM or equivalent part is used. Note that some internal transmission speed sensors may require transmission removal for access.
• Check and Replace Fuses: If a specific fuse for the transmission sensor power circuit is found blown, replace it. However, always investigate the cause of the blown fuse (e.g., a shorted wire or component) to prevent immediate recurrence.
• Replace the PCM/TCM: If all wiring, connectors, and sensors test definitively good, and the PCM/TCM is confirmed not to be supplying the correct reference voltage to the sensor power circuit, then the module itself is likely faulty. This is a less common but possible scenario. After replacement, ensure the module is properly programmed or re-learned according to manufacturer specifications.
• Mechanic’s Tip: Always clear the DTCs after any repair and perform a road test under varying conditions to confirm the fix and ensure the code does not return. Pay attention to vehicle-specific technical service bulletins (TSBs) for known issues related to this code or transmission sensor circuits for the specific make and model.