P0840
UniversalTransmission Fluid Pressure Sensor/Switch A Circuit
Medium SeverityThe P0840 code indicates an issue with the electrical circuit for the Transmission Fluid Pressure Sensor/Switch 'A'. This typically means the powertrain control module (PCM) is not receiving a correct signal from the sensor, which can lead to improper gear shifts or delayed engagement of the transmission.
⚠️ Before You Begin (Safety First)
Always ensure the vehicle is safely supported on jack stands if working underneath. Disconnect the battery before performing any electrical work. Allow the engine and transmission to cool down before handling hot fluids or components.
Common Symptoms
- Check Engine Light illuminated
- Delayed or harsh shifting
- Transmission slipping
- Transmission stuck in 'limp mode'
- Lack of power
- Transmission may not shift into certain gears
Common Causes
- Faulty Transmission Fluid Pressure Sensor/Switch A
- Damaged or corroded wiring or connectors in the sensor circuit
- Low transmission fluid level
- Contaminated transmission fluid
- Internal transmission mechanical issue (less common for circuit code)
- Faulty Powertrain Control Module (PCM) (rare)
Frequently reported on: GM Vehicles (Early 2000s-2010s), Ford Trucks (2004-2014), Honda Vehicles (2000s), Chrysler/Dodge Vehicles (Late 2000s-Early 2010s), Nissan Vehicles (2000s)
Diagnostic Steps
- Check transmission fluid level and condition according to manufacturer specifications.
- Visually inspect the wiring and connectors leading to the Transmission Fluid Pressure Sensor for any signs of damage, corrosion, or loose connections.
- Consult a service manual for the specific location of Sensor/Switch 'A' and its circuit diagram.
- Use a multimeter to test for continuity and proper voltage at the sensor connector (if comfortable and knowledgeable with electrical testing).
- If wiring and fluid are good, replace the Transmission Fluid Pressure Sensor/Switch A.
- Clear the code and test drive the vehicle to verify the repair.
👨🔧 When to Call a Professional
If the vehicle is experiencing severe transmission issues like harsh shifting, slipping, or refusal to engage gears, do not drive it further and have it towed to a professional mechanic immediately to prevent further damage.
Commonly Asked Questions
Can I drive with a P0840 code?
It's not recommended to drive for an extended period, especially if you experience noticeable shifting problems. Driving with a faulty sensor can lead to further transmission damage.
Where is the Transmission Fluid Pressure Sensor 'A' located?
The exact location varies by vehicle, but it's typically found inside the transmission valve body or mounted on the outside of the transmission case. Consult your vehicle's service manual for precise location.
Is it possible for low transmission fluid to cause a P0840?
Yes, low or contaminated transmission fluid can directly affect the pressure readings the sensor provides, potentially triggering this code. Always check the fluid level and condition first.
Do I need special tools to replace the sensor?
You'll typically need basic hand tools. Some sensors might require a specific wrench or socket. If it's located inside the transmission, you'll need to drop the transmission pan, requiring new fluid and a pan gasket.
What if replacing the sensor doesn't fix the code?
If the new sensor doesn't resolve the issue, the problem might be with the wiring harness, the electrical connector, internal transmission issues, or, less commonly, the powertrain control module (PCM). Further diagnostics are needed.