Transmission Fluid Pressure Sensor/Switch A Circuit High
Diagnostic trouble code P0843 indicates that the powertrain control module (PCM) or transmission control module (TCM) has detected a high voltage signal from the transmission fluid pressure sensor/switch 'A' circuit. This means the sensor's output voltage is exceeding the maximum expected threshold for the current operating conditions, suggesting a fault in the sensor, its wiring, or internal transmission pressure.
While the vehicle may still be drivable, it should be diagnosed and repaired soon. Ignoring this code can lead to harsh shifting, transmission slipping, and potential internal transmission damage over time.
Common symptoms
- Illuminated Check Engine Light
- Harsh or erratic transmission shifting
- Transmission slipping between gears
- Vehicle stuck in 'limp mode' (restricted to a single gear)
- Decreased fuel efficiency
Likely causes
- Defective transmission fluid pressure sensor/switch 'A'
- Low or dirty transmission fluid
- Damaged, corroded, or shorted wiring in the sensor circuit
- Poor electrical connection at the sensor connector
- Internal transmission mechanical failure or valve body issue
- Faulty PCM or TCM (rare)
What to check before replacing parts
Check the transmission fluid level and condition.
Look for: Fluid that is significantly low, burnt, or heavily contaminated with debris indicates a fluid issue that could cause pressure problems or sensor malfunction.
Visually inspect the wiring harness and connector for the transmission fluid pressure sensor 'A'.
Look for: Look for frayed wires, melted insulation, corrosion in the connector pins, or a loose connection that could cause a high resistance or short to voltage.
Scan for other transmission-related DTCs.
Look for: The presence of other codes (like shift solenoid or gear ratio codes) can help determine if the issue is isolated to the sensor circuit or if there is a broader internal transmission problem.
Test the transmission fluid pressure sensor 'A' circuit for voltage and ground using a multimeter.
Look for: With the key on, engine off, check for a 5-volt reference and a good ground. A reading of battery voltage on the signal wire when it shouldn't be there indicates a short to power.
Test the transmission fluid pressure sensor 'A' resistance or output voltage while operating.
Look for: Compare the sensor's resistance or live data voltage output against the manufacturer's specifications. If the voltage remains pegged high regardless of actual pressure, the sensor is likely faulty.
This page explains the generic P0843 definition and is general reference information, not a diagnosis for your specific vehicle. Actual causes, severity, and repairs vary by make, model, engine, and mileage — always confirm with a scan tool and a qualified technician before replacing parts.
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