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DTC codes / P08xx · Transmission / P0873

P0873Transmission Fluid Pressure Sensor/Switch 'C' Circuit High

P0873 means the transmission control module (TCM) sees the signal voltage from transmission fluid pressure sensor or switch 'C' stuck above its normal range, so it cannot trust that pressure reading. The computer compares this circuit against expected values and flags a high-voltage condition, which usually points to an open circuit, a short to voltage, or a failed sensor rather than low fluid pressure itself.

SystemPowertrain
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyIntermediate
AreaTransmission

Symptoms you may notice

  • Illumination of the check engine light or transmission warning light, often with no other symptoms besides the warning light
  • Harsh or delayed shifts as the transmission defaults to a limp or fixed-pressure strategy
  • The transmission may stay in a single gear or refuse to downshift when you slow down
  • A noticeable flare or slip when accelerating because line pressure is no longer being trimmed by sensor feedback
  • In some vehicles, the engine may derate or the torque converter clutch may stop engaging
  • No drivability change at all if the fault is intermittent and the TCM has not yet entered limp mode

Common causes, most likely first

  1. An open circuit in the sensor 'C' signal wire, a broken wire, or a disconnected connector at the transmission
  2. A short to voltage on the signal circuit, often from chafed harness insulation near hot or moving components
  3. A failed transmission fluid pressure sensor or switch 'C' that reads high or has an internal open element
  4. Corroded, pushed-back, or oil-contaminated pins in the transmission connector or TCM connector
  5. A poor ground or reference voltage fault shared by the pressure sensor circuit
  6. A faulty TCM that misreads a correct sensor signal, though this is the least common cause
  7. Low or aerated transmission fluid that changes the pressure signal behaviour, though this usually sets a different code first

How to diagnose and fix it

  1. Scan all modules and record freeze-frame data for P0873, then check for related pressure-sensor and shift-solenoid codes before touching anything
  2. Inspect the transmission harness and connector for chafing, corrosion, oil wicking, and loose pins, and repair any damaged wiring
  3. Check transmission fluid level and condition per the manufacturer's procedure, since low or burnt fluid can distort pressure readings
  4. Back-probe the sensor 'C' signal circuit and compare voltage with the manufacturer's specification at key-on and at idle
  5. Measure resistance and continuity of the sensor 'C' circuit from the TCM connector to the sensor connector to isolate an open or short
  6. If the circuit tests good, replace the transmission fluid pressure sensor or switch 'C' and clear the code
  7. After repair, clear codes, perform any required transmission adaptive relearn, and road-test through all gears to confirm the code does not return

What the high-voltage reading really means

  • A high-voltage code almost never means high transmission fluid pressure; it means the signal circuit is open or shorted to voltage, so the TCM sees a voltage above the normal range.
  • Technicians often replace the sensor first and still have the code because the real fault is a chafed signal wire inside the transmission harness near a hot exhaust component.
  • A disconnected or partially seated transmission connector after recent service is a very common cause that looks exactly like a failed sensor on the scan tool.
  • Some vehicles share the pressure sensor reference voltage with other sensors, so a fault in a neighbouring circuit can pull the 'C' signal high and set P0873 without any problem in the 'C' circuit itself.
  • If the code is intermittent and the transmission shifts normally, inspect the harness for movement-related damage before condemning any hard part.

Can I keep driving with P0873?

You can usually drive the vehicle to a shop if the transmission still shifts and the light is steady, but avoid heavy acceleration, towing, and long highway trips because the transmission may be running on default pressure. If the vehicle enters limp mode, flares badly, or refuses to shift, stop driving and have it towed to prevent internal transmission damage.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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