P0840 - Transmission Fluid Pressure Sensor/Switch "A" Circuit

The TCM saw a transmission hydraulic pressure signal outside the expected range, pointing to a pressure sensor fault, wiring issue, low fluid pressure, or an internal transmission problem.

Code first. Confirm symptoms and checks before pricing parts.

Code Overview

The TCM saw a transmission hydraulic pressure signal outside the expected range, pointing to a pressure sensor fault, wiring issue, low fluid pressure, or an internal transmission problem.

Common Causes

• Faulty pressure sensor or switch
• Low or dirty transmission fluid
• Harness or connector damage
• Internal hydraulic or valve-body issue

Diagnostic Steps

1. Check transmission fluid level and condition
2. Inspect the transmission connector and harness for oil intrusion or damage
3. Compare sensor readings to commanded values with a scan tool
4. Do not condemn the transmission before verifying power, ground, and reference signals

Likely Repairs & Cost Guides

Confirm the repair path before pricing parts.

Related OBD Codes

Browse nearby codes when systems overlap.

Next Steps

Move from code to checks, then estimate.