P0843 Code: Transmission Fluid Pressure Sensor A (Stop Guessing & Fix)

Picture this: You are pulling away from a stoplight. Instead of a smooth shift into second gear, your car violently bangs into gear, spilling your coffee. You look down at the dashboard, the Check Engine Light is on, and the "D" (Drive) indicator is flashing uncontrollably. The car now feels completely gutless, stuck in Limp Mode.

If your scanner pulls code P0843, don't panic. Some shady repair shops will immediately quote you $3,500 for a full transmission rebuild. The truth? It might just be a broken wire or a $30 sensor on the outside of the transmission.

Flashing D drive light on dashboard with P0843 code

A flashing Drive indicator and harsh shifting are terrifying, but they are often caused by a simple external electrical fault, not catastrophic internal failure.

What Does Code P0843 Mean?

The official definition for P0843 is "Transmission Fluid Pressure Sensor/Switch 'A' Circuit High".

Automatic transmissions use hydraulic fluid pressure to squeeze clutch packs together and shift gears. The Transmission Control Module (TCM) needs to know if the correct pressure is actually reaching those clutches. It does this using a Fluid Pressure Switch (Sensor "A").

Code P0843 is strictly an electrical code. "Circuit High" usually means the computer is seeing a constant 5 volts on that wire. In the automotive world, an abnormally high voltage reading almost always means the circuit is "open"—meaning the sensor is unplugged, the wire is completely broken, or the internal mechanism of the sensor itself has snapped.

Symptoms & Root Causes: Why Did It Fail?

When the computer loses the signal from this sensor, it assumes the worst and maxes out the transmission fluid line pressure to prevent the clutches from slipping. This is why you experience:

Violent, harsh shifting (especially from 1st to 2nd or 2nd to 3rd).
Transmission locked in Limp Mode (usually stuck in 3rd gear).
Flashing "D" or "Drive" light (extremely common on Honda and Acura models).

Before you even think about dropping the transmission pan, check these common culprits:

Faulty Pressure Switch (Most Common): The sensor takes a beating from hot transmission fluid and engine vibration. The internal diaphragm eventually cracks, breaking the electrical connection.
Broken or Rodent-Chewed Wiring: Because these sensors sit low in the engine bay, their thin wires are notorious targets for road debris or hungry mice, causing an open circuit.
Unplugged Connector: If you recently had an oil change or engine work done, the mechanic might have accidentally bumped the connector off the sensor.

Pro Tech Alert: If you drive a Honda Accord, Civic, CR-V, or Odyssey, P0843 is almost exclusively a failed 2nd clutch transmission fluid pressure switch. It is located right on the outside of the transmission case. You can replace it in your driveway with a single wrench in 10 minutes without draining the fluid.

Stop Guessing. Interrogate the TCM.

Is the sensor dead, or is a wire broken somewhere deep in the harness? You can find out without a multimeter. The iCarsoft CR MAX P gives you direct access to the Transmission Control Module Live Data.

Plug the scanner in and look at the "Pressure Switch A" data stream. It will read either ON or OFF. Unplug the sensor and bridge the harness pins with a paperclip. If the scanner instantly flips the reading from OFF to ON, your wiring and computer are perfect—the sensor is 100% dead. Prove it before you pay a diagnostic fee.

See the CR MAX P in Action

Mechanic using iCarsoft CR MAX P scanner to read transmission fluid pressure switch live data

Stop guessing. Viewing the live ON/OFF state of the pressure switch through the TCM proves whether you have a bad sensor or a broken wiring harness.

Step-by-Step Fix: How to Diagnose Code P0843

Here is the exact diagnostic path to follow to save yourself thousands of dollars:

Step 1: The Visual Inspection. Locate Sensor A on your transmission (consult a repair manual or forum, as it varies by make). Ensure the plug is firmly seated. Pull the plug off and look for green corrosion or transmission fluid leaking through the sensor into the connector (a sure sign of failure).
Step 2: The Jumper Wire Test. With the ignition ON (engine OFF), unplug the sensor. Use a paperclip to jump the signal wire in the harness to a good metal ground on the engine. If your scanner's live data changes state, or the code changes from P0843 (Circuit High) to a "Circuit Low" code, the wiring is good. Replace the sensor.
Step 3: Replace the Sensor. This is usually a 22mm or 24mm wrench size. Simply unscrew the old sensor and screw the new one in. You will only lose a few drops of transmission fluid. Make sure to use a new crush washer if required.

Replacing faulty automatic transmission fluid pressure switch with a wrench

On most vehicles, you do not need to drop the transmission pan or drain the fluid to replace a pressure switch. It threads right into the outside of the case.

FAQ: What Else You Need to Know

Can I drive my car with a P0843 code?

It is not recommended. While the car will physically move, the transmission computer is commanding maximum line pressure to prevent slipping. This causes the incredibly harsh shifts, which put massive mechanical stress on your axles, motor mounts, and the internal transmission bands.

Will a transmission fluid flush fix the P0843 code?

No. "Circuit High" is an electrical fault, meaning the circuit is broken or the sensor has failed internally. Changing the fluid will not fix a broken wire or a shattered sensor diaphragm. Fix the electrical issue first.

How much does it cost to fix code P0843?

If you do it yourself, the OEM pressure switch typically costs between $30 and $60. It takes about 15 minutes to install. If you go to a dealership, expect to pay $150 for a diagnostic fee, plus marked-up parts and labor, bringing the total to $250 - $400 for a very simple repair.