OBD-II trouble code
P0840: Transmission Fluid Pressure Sensor/Switch "A" Circuit
A general electrical fault was detected in the transmission fluid pressure sensor or switch 'A' circuit. Usually wiring or a failed sensor, though it can occasionally reflect a genuine pressure problem — verify before replacing parts.
Quick facts
- System
- Powertrain
- Category
- Transmission / Pressure Control
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $120 – $600
- DIY difficulty
- Intermediate DIY
What does P0840 mean?
P0840 sets when the transmission control module (TCM) or PCM detects a general fault in the circuit for transmission fluid pressure sensor/switch "A." This is the broad, non-directional version of the pressure-sensor codes: it flags that something is wrong with the circuit without specifying low, high, or range/performance the way P0841, P0842, and P0843 do.
The sensor (or switch) reports the hydraulic pressure inside the transmission so the module can confirm the clutches are being applied with the right force and time its shifts. When the module can't trust that signal, it may set P0840 and, depending on the transmission, adjust shift strategy or drop into a fail-safe limp mode to protect the clutches.
Most of the time P0840 is an electrical problem: a corroded or loose connector, chafed or shorted wiring, or a failed sensor. The transmission connector lives in a hot, fluid-exposed environment, so corrosion and harness damage are common. Less often, the circuit fault sits alongside a genuine hydraulic issue — so if the wiring checks out but shifts feel wrong, confirm actual line pressure with a gauge rather than assuming the sensor is simply lying. Because the code is generic, diagnosis leans on inspecting the circuit thoroughly and comparing the sensor's live reading against a mechanical pressure gauge.
Common causes
- Corroded, loose, or damaged transmission connector
- Chafed, pinched, open, or shorted wiring in the sensor circuit
- Failed transmission fluid pressure sensor/switch A
- Poor sensor ground or reference voltage
- Contaminated fluid affecting an in-pan sensor
- Genuine hydraulic pressure fault accompanying the circuit code (less common)
Symptoms
- Check engine light on with P0840 stored
- Harsh or poorly timed shifts
- Transmission entering limp mode in some cases
- Erratic shift behavior
- Often no strong driveability symptoms if pressure is actually fine
Diagnostic steps
- 1.Scan for companion codes; a directional sibling like P0842/P0843 or a pressure code like P0868 helps narrow the fault.
- 2.Inspect the transmission connector and sensor wiring for corrosion, chafing, opens, and shorts.
- 3.Back-probe the sensor signal and check voltage against specification with the key on and engine running.
- 4.Verify the sensor's reference voltage and ground are present and clean.
- 5.Substitute or bench-test the pressure sensor if the wiring checks out.
- 6.If the circuit is healthy, measure actual line pressure with a gauge to rule out a genuine hydraulic problem.
Repair cost
$120 – $600
A wiring or connector repair is often $100-$250. Replacing the transmission fluid pressure sensor runs roughly $150-$450 depending on whether it's externally mounted or inside the pan on the valve body. If the fault turns out to reflect a genuine hydraulic problem, the cost follows whatever pressure repair is needed.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with transmission shift solenoid replacement preselected. Adjust labor rate and vehicle category to fit your situation.
DIY vs shop
This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.