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DTC codes / P08xx · Transmission / P0842

P0842Transmission Fluid Pressure Sensor/Switch 'A' Circuit Low

P0842 means the transmission control module (TCM) has detected a low-voltage condition on the signal circuit from transmission fluid pressure sensor or switch 'A'. The sensor monitors hydraulic pressure inside the transmission, and the TCM compares the reported voltage against expected values for the current operating conditions; a sustained low reading triggers this code and often forces the transmission into a default strategy.

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, especially when the transmission is cold or under light throttle
  • The transmission may enter limp mode, holding a single gear or refusing to downshift
  • Slipping or flaring between gears as line pressure control is compromised
  • Erratic torque converter clutch engagement or shudder at steady cruise
  • In some vehicles, the PCM may command reduced engine torque, causing sluggish acceleration

Common causes, most likely first

  1. A chafed, corroded, or disconnected wire in the sensor 'A' signal circuit, especially near the transmission case connector
  2. A failed transmission fluid pressure sensor or switch 'A' itself, reading low or open internally
  3. Low transmission fluid level or degraded fluid that prevents the sensor from seeing correct hydraulic pressure
  4. A sticking or leaking valve body solenoid or pressure regulator that starves the sensor port of pressure
  5. Poor terminal tension or water intrusion at the TCM harness connector
  6. A faulty TCM driver circuit that misreads a normal sensor signal as low voltage

How to diagnose and fix it

  1. Check transmission fluid level and condition first, topping off or servicing fluid if it is low or burnt
  2. Inspect the sensor 'A' harness and connector for chafing, corrosion, or loose pins, repairing as needed
  3. Use a scan tool to read live transmission pressure sensor voltage and compare it to the manufacturer's expected range at idle and in gear
  4. Backprobe the sensor signal circuit at the TCM to verify the low voltage is real and not a wiring drop
  5. Test the sensor or switch 'A' with a multimeter for resistance or voltage output against specification, replacing it if out of range
  6. Check valve body line pressure with a gauge to confirm the hydraulic circuit is not the true cause of the low reading
  7. Clear the code and perform a relearn or adaptation drive cycle, then verify the code does not return

What a low pressure reading really means

  • A low signal voltage often comes from a poor ground or reference circuit shared with other sensors, so testing the sensor in isolation can miss the real fault.
  • The code can set intermittently from a wiring harness that rubs only during certain engine or transmission movements, which is why a wiggle test of the harness is essential.
  • Low fluid level or a clogged filter can produce this code without any electrical fault, because the sensor accurately reports low pressure that the TCM interprets as a circuit problem.
  • Some vehicles use a switch rather than an analog sensor for 'A', so a scan tool may only show open or closed; a low circuit then means the switch is stuck open or unplugged.
  • Replacing the sensor without checking line pressure risks a repeat code if a worn pump or leaking valve body is the true source of the low reading.

Can I keep driving with P0842?

You can usually drive the vehicle to a repair shop, but avoid heavy throttle, towing, or long highway trips because the transmission may be in limp mode and shifting harshly. If the transmission slips badly or refuses to move, stop driving and have it towed to prevent internal 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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