Transmission Fluid Pressure Sensor/Switch A Circuit
P0840 means transmission fluid pressure sensor/switch a circuit. The ECM saw a value or response outside its calibrated window for the transmission system. Real-world fixes usually start with the highest-probability cause below for your year, make, and model - not a parts-cannon guess.
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Electrical range or stuck solenoid produces harsh/delayed shifts.
#2Fluid level, condition, or filter$80-$350
0%
Burnt fluid or low level drops line pressure and trips sensors.
#3Wiring/TCM power or ground$50-$400
0%
Low TCM voltage or open circuits fake sensor faults.
#4Internal clutch pack or valve body wear$1200-$4500
0%
Mechanical slip exceeds adaptive control limits.
Symptoms Radar
Check engine light on (or flashing if misfire-related)
Reduced power, limp mode, or hard starting
Failed emissions readiness or inspection risk
Harsh, delayed, or slipping shifts
Related codes stored with freeze-frame data
Repair Cost
DIY
$80-$450
Shop
$250-$1800
Diagnose It
1Confirm P0840 is current (not history-only) and save freeze-frame RPM, load, and fuel trim.
2Inspect connectors and harness for the listed sensor/actuator before replacing parts.
3Command the actuator or graph the sensor live with a scan tool; compare to known-good values.
4Fix the root cause, clear codes, and verify monitors complete on a mixed drive cycle.
FAQ
Can I drive with a P0840 code?
Park it if power is cut, shifting is violent, or the light is flashing - short trips only if it still drives normally.
How much does it cost to fix P0840?
DIY parts typically land around $80-$450. A shop diagnosis plus repair commonly runs $250-$1800, depending on whether it is a sensor, harness, or major component.
What is the most common cause of P0840?
Shift or pressure control solenoid is the most common starting point (~38%), followed by fluid level, condition, or filter.