P0840 — Transmission Fluid Pressure Sensor/Switch A Circuit

P0840: Transmission Fluid Pressure Sensor/Switch A Circuit — what it means, the affected system and what to check before replacing parts.

Meaning, symptoms, causes and checks

Transmission Fluid Pressure Sensor/Switch A Circuit. Basic catalogue definition; diagnostic details have not yet been reviewed.

Affected system: Powertrain

Symptoms

  • The transmission may log an abnormal A fluid-pressure sensor/switch signal (faulty or implausible).
  • A warning and harsh, delayed or restricted gear changes are possible; some vehicles show little change.

Possible causes

  • An unreliable pressure sensor, damaged connector or wiring can produce an implausible reading.
  • Incorrect fluid condition or a hydraulic control problem may also affect the measured pressure; the code alone cannot identify the failed part.

What to check

  • Read associated transmission faults and live pressure-sensor information; note operating conditions.
  • Check fluid level/condition by the transmission procedure and inspect the sensor wiring; compare sensor readings with the maker's expected values before replacing components.

Also written as: P0840-00, P084000, PO840 (some scan tools add the fault-type byte, e.g. -00).