Comprehensive diagnostic guide for OBD-II code P0874

Overview

• P0874 generally corresponds to a Transmission Fluid Pressure Sensor/Switch C Circuit Range/Performance issue, reported as an intermittent signal (Sensor C) for the transmission fluid pressure circuit. The open-source code description explicitly identifies it as an intermittent signal from the Sensor C of transmission fluid pressure. In practice, this DTC points to an intermittent or out-of-range reading on the C circuit of the transmission fluid pressure sensor/switch.
  • Open Source reference: Sinal intermitente do sensor/interruptor C de pressão fluido transmissão (transmission fluid pressure sensor C intermittent signal) for P0874.
  • Wikipedia context: P0874 is a Powertrain/OBD-II diagnostic trouble code (P0xxx family) in the broader body of Powertrain Codes and Diagnostic Trouble Codes used by modern vehicles to monitor parameters and trigger codes when issues are detected.
  • Sources: OBD-II Diagnostic Trouble Codes, Powertrain Codes. Also, the interpretation from the open-source listing aligns with the general "intermittent signal" description.

What you might experience (symptoms)

• Intermittent or sporadic transmission behavior related to hydraulic pressure sensing:
  • Erratic or harsh shifting, delayed shifts, or unexpected upshifts/downshifts under certain temps or loads.
  • Occasional transmission performance loss or limp/limited operation that seems tied to pressure readings.
  • MIL illumination or a stored P0874 in the ECM/TCM memory, sometimes with other related codes.
• These symptoms reflect the sensor reporting out-of-range or inconsistent data to the transmission control logic, which can affect hydraulic target pressures and shift profiles.
• Note: The above symptom pattern is consistent with the nature of a circuit range/performance DTC, and aligns with the general role of transmission pressure sensors in controlling shift quality.

Probable Causes and the explanatory description of "intermittent signal" for Sensor C, with no NHTSA-specific frequency data available.

• Faulty or intermittently failing Transmission Fluid Pressure Sensor C (sensor itself or its internal conditioning): ~45%
• Wiring harness issues, damaged/abnormal connections, or poor grounding on the Sensor C circuit (including corrosion, pin tension, pin spread): ~30%
• PCM/ECU input, wiring harness harness routing to the PCM, or software calibration issue affecting the Sensor C signal interpretation: ~15%
• Actual hydraulic/transmission mechanical condition causing pressure readings to be out of spec : ~10%

Notes:

• The above distribution reflects the emphasis on intermittent sensor signal and common failure modes (sensor, wiring, PCM). If you find a mechanical hydraulic issue that consistently produces out-of-range pressure, treat it as a separate but related cause pathway and verify with actual pressure measurements if accessible.

Tools Needed

• OBD-II scan tool with live data capability to monitor Sensor C readings/voltage, reference (5V) supply, and ground status; check for freeze-frame data and any related codes.
• Digital multimeter (DVOM) for:
  • Supply voltage to Sensor C (reference/5V line) and ground continuity.
  • Signal wire voltage with engine running, and signal stability during engine RPM/temperature changes.
• Optional: oscilloscope to visualize sensor C waveform (look for noise, spikes, or erratic transitions).
• Wiring and connector inspection tools:
  • Good lighting, magnification if needed, magnifier for pins, and a multimeter to check continuity in the Sensor C circuit from the sensor to the PCM.
• If available and vehicle-specific, a transmission hydraulic pressure test port or a pressure gauge to corroborate sensor readings with actual hydraulic pressure (to distinguish sensor fault from hydraulic issue).

Diagnostic Approach

1) Confirm and document the DTC

• Use an OBD-II scanner to confirm P0874 is current or in history, and capture the freeze-frame data (engine RPM, transmission temperature, gear, road speed if stored).
• Note any related codes (especially other P0xxx or P087x family codes) that could point to a common wiring harness issue or a separate pressure sensor circuit issue (A, B, C, etc.).

2) Visual inspection and basic condition check

• Inspect the Transmission Fluid level and condition (appropriate level, clean fluid, no burnt smell). Fluid condition can influence hydraulic behavior and pressure transients.
• Inspect Sensor C wiring harness routing, connectors, and sensor ground/grounding points for signs of wear, abrasion, corrosion, moisture intrusion, or loose connections.
• Check for bent pins, missing seals, or misaligned connectors on the Sensor C connector.

3) Electrical checks on Sensor C circuit (power, ground, signal)

• Power/Reference: With ignition ON (engine OFF if possible, according to service safety guidelines), backprobe or probe the Sensor C reference voltage from the PCM and verify it is near the expected value (commonly around 5V for many sensor circuits). Check for voltage drop or instability.
• Ground: Verify Sensor C ground continuity to chassis/PCM ground. Any intermittent grounding can cause signal instability.
• Signal (Sensor C output): With the engine running and vehicle in a range where the code previously appeared, monitor the Sensor C signal voltage. Look for:
  • Voltage that is consistently out of expected range (too high or too low).
  • Signal noise, spikes, or rapid fluctuations that correspond with engine/transmission events or vibrations.
  • Intermittent losses of signal when wiggling connectors or harnesses (perform a controlled wiggle test on the Sensor C harness and connector to reproduce the fault).
• Short-to-supply or short-to-ground: Check for any shorts on the signal line that could cause intermittent out-of-range readings.

4) Cross-check with live transmission data

• If the scanner supplies live data for transmission pressure-related parameters, compare Sensor C readings with the transmission's actual hydraulic pressure (if you have a way to view or measure actual pressure). Significant discrepancies between Sensor C and actual pressure readings indicate a sensor or wiring fault; close agreement with actual mechanical pressure inside the system may point toward a pressure/valve issue rather than the sensor itself.

5) Rule out related control/communication issues

• If Sensor C and its wiring appear sound, consider potential PCM/ECU issues or software calibration that could misinterpret the sensor signal. Check for vehicle-specific service bulletins (TSBs) related to Sensor C or P0874.

6) Mechanical/hydraulic considerations (if sensor and wiring test out fine)

• Acknowledge that hydraulic issues can present as out-of-range readings on the sensor even if the sensor is healthy. If accessible, verify transmission hydraulic pressure through diagnostic ports or temporary pressure measurement (where permitted by the vehicle and shop procedures). Consider valve body or solenoid concerns if pressure readings are consistently off but sensor data is clean.

7) Repair or replacement steps (based on root cause)

• Sensor C fault or intermittent signal:
  • Replace Sensor C with the correct OEM or equivalent sensor, following the vehicle's service procedure.
  • Inspect/repair the Sensor C connector and harness traces, ensuring proper pin fitment and sealing.
• Wiring/connector fault:
  • Repair or replace damaged wiring, restore intact insulation, and ensure secure, corrosion-free connections. Clean terminals if applicable. Re-secure harnesses away from heat sources or moving parts.
• PCM/ECU issue:
  • If wiring checks out and the sensor functions properly, consider reprogramming or updating the PCM/ECU software per manufacturer TSBs or service bulletins. Clear codes after repair and re-test.
• Hydraulic/mechanical transmission issue:
  • If sensor and wiring tests are good but actual hydraulic pressures are abnormal, investigate transmission internal components (valve body, solenoids, pump, seals) as per service procedure. This is less likely the direct cause of P0874 if the sensor is healthy, but still possible if actual pressure is out of spec due to mechanical faults.

Post-Repair Verification

• Clear P0874 and any related codes after repairs.
• Take a road test (drive cycle that exercises the conditions under which the fault previously occurred) and re-scan to confirm the code does not return.
• Monitor Sensor C during the test drive to ensure readings are within a stable, expected range and no intermittent signal is detected.

Safety Considerations

• Follow standard engine and transmission service safety practices: allow fluids to cool before touching hot components; use appropriate protective gear; ensure the vehicle is properly supported if raised; disconnect battery power when performing certain electrical tests per manufacturer guidelines.
• Avoid introducing moisture or contaminants into electrical connectors during inspection or repair.
• If diagnosing hydraulic pressure, be mindful of pressurized systems and safe handling of transmission fluid.

What to document and reference

• Findings: exact Sensor C voltage readings, reference voltage, ground continuity results, observed intermittent behavior during wiggle tests, and any correlation with engine/transmission conditions.
• Code context: whether P0874 is current vs. history, and any related codes observed.
• Repair steps performed and test results, including test drive outcomes.

This diagnostic guide was generated using verified reference data:

• Wikipedia Technical Articles: OBD-II
• Open-Source OBD2 Data: N/A (MIT)

Content synthesized from these sources to provide accurate, real-world diagnostic guidance.

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