P0840 Generic (SAE) Basic entry

Transmission Fluid Pressure Sensor/Switch 'A' Circuit

Updated 2026-10-02 · Transmission

Severity

Medium

Typical repair cost

60 – 500 EUR

System

Powertrain

What it means

P0840 is defined as "Transmission Fluid Pressure Sensor/Switch 'A' Circuit". the engine control module stores it when the monitored condition for this component or system is outside the limits programmed by the manufacturer.

This entry is compiled automatically from the standard definition and typical diagnostic practice for this type of fault. A detailed, reviewed article is in preparation.

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Symptoms

Check engine or transmission warning light
Harsh, delayed or missing shifts
Transmission stuck in one gear (limp mode)
Shuddering or slipping under load

Common causes

Transmission Fluid Pressure Sensor/Switch 'A' Circuit worn, contaminated or out of calibration
Mechanical problem in the system the sensor measures
Air, vacuum or exhaust leak influencing the reading
Wrong or low-quality replacement part
Wiring resistance causing a small voltage offset

How to diagnose

Compare the live Transmission Fluid Pressure Sensor/Switch 'A' Circuit reading with a known-good value or a second sensor at idle and at 2500 rpm
Check for air, vacuum or exhaust leaks that could skew the reading
Inspect the component for contamination and clean it if the design allows
Verify the part number is correct for the engine
Replace the Transmission Fluid Pressure Sensor/Switch 'A' Circuit if the reading remains out of specification

Frequently asked questions

Can I keep driving with P0840?

Usually yes for a short time, but expect reduced performance or higher fuel use. Have it diagnosed within a few days.

Will clearing P0840 fix it?

No. Clearing only erases the stored code; it returns as soon as the monitor runs again unless the cause is repaired.