P0843
Transmission Fluid Pressure Sensor/Switch A Circuit High
If your vehicle's onboard computer has flagged the diagnostic trouble code P0843, it refers to a detected anomaly regarding "Transmission Fluid Pressure Sensor/Switch A Circuit High". This systemic engine fault needs a targeted check before symptoms expand.
Driver's Summary
The diagnostic trouble code P0843 indicates an active fault in the transmission fluid pressure sensor/switch a circuit high circuit or component. In practice, this fault causes limp mode, erratic upshifts, mil illuminated. Given the high severity of this code, continuing to drive risks significant mechanical damage. Have it diagnosed immediately.
Symptoms
Limp mode, erratic upshifts, MIL illuminated
Common Causes
- Short to battery voltage in TFP sensor A circuit
- Failed TFP sensor
- Broken ground wire
- Excessive line pressure due to stuck valve
How to Fix
- 1 Repair short to voltage
- 2 Replace Transmission Fluid Pressure Sensor
- 3 Fix ground connection
- 4 Clean or rebuild valve body
Technical Explanation
The ECM detects code P0843 by continuously monitoring the relevant sensor circuit against calibrated threshold values stored in its non-volatile memory. The TCM compares the ratio between input turbine speed sensor and output speed sensor readings against the expected gear ratio stored for each commanded gear position. A deviation greater than a few percent indicates clutch slippage, solenoid malfunction, or internal mechanical failure. The fault remains stored in memory even after the MIL is cleared; it becomes a confirmed DTC after failing two consecutive drive cycles, and the PCM logs a freeze frame record of the engine's exact operating state at the moment of detection.
Is It Safe to Drive?
With P0843 active, your engine or transmission is not operating within design parameters. Short-term driving may seem fine, but internal damage is accumulating — particularly to short to battery voltage in tfp sensor a circuit.
Mechanic's Pro Tip
The most common mistake with P0843 is replacing the sensor without verifying the reference voltage and ground integrity first. Use a scan tool to monitor the sensor's live output; a truly failed sensor shows a stuck, flatlined reading — a sensor that fluctuates but reads slightly off usually indicates a wiring or vacuum issue, not a dead sensor. Always spray electrical contact cleaner on the connector pins before condemning the sensor.
Sensor: $150 - $400; Valve body work: $400 - $800