OBD-II trouble code
P0842: Transmission Fluid Pressure Sensor/Switch "A" Circuit Low
The transmission fluid pressure sensor or switch 'A' is sending a signal that's too low — a low voltage the control module reads as out of range. Usually a wiring or sensor problem, though it can reflect genuinely low pressure.
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 P0842 mean?
P0842 sets when the transmission control module (TCM) or PCM reads the signal from transmission fluid pressure sensor/switch "A" as too low — a voltage below the expected range for the circuit. This 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 accordingly. "Circuit Low" points at the electrical signal being low, not necessarily the pressure itself.
Most of the time this is an electrical fault. A short to ground in the signal wire, a corroded or loose connector, or a failed sensor will pull the reading low and set the code. Because the transmission connector lives in a harsh, hot, fluid-exposed environment, corrosion and chafed wiring are common. A sensor that has failed internally can also latch the signal low.
That said, the code can occasionally reflect reality — if line pressure is genuinely low, the sensor will honestly report a low value. That's why it's worth cross-checking: if the electrical circuit tests clean but the reading is still low, confirm actual pressure with a gauge before assuming the sensor is lying. When P0842 appears alongside a pressure code like P0868, the low reading may be real. On its own, with good pressure, it's almost always wiring or the sensor.
Common causes
- Short to ground in the pressure sensor signal wire
- Corroded, loose, or damaged transmission connector
- Failed transmission fluid pressure sensor/switch A
- Chafed or pinched wiring in the sensor circuit
- Poor sensor ground or reference voltage
- Genuinely low line pressure being honestly reported (less common)
Symptoms
- Check engine light on with P0842 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 P0868 alongside P0842 suggests the low reading may reflect real pressure.
- 2.Inspect the transmission connector and sensor wiring for corrosion, chafing, and shorts to ground.
- 3.Back-probe the sensor signal and check the voltage against specification with the key on.
- 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 reading turns out to reflect genuinely low pressure, the cost follows whatever hydraulic 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.