Transmission Fluid Pressure Sensor/Switch A Circuit Low
Your transmission fluid pressure sensor is sending a signal that's too weak or absent, like a dimmer switch stuck on low. The transmission can't properly monitor its hydraulic pressure, risking shifting problems or damage.
The ECU monitors voltage output from the transmission fluid pressure sensor, which varies with hydraulic pressure. When voltage drops below the normal threshold (typically 0.5V), the ECU interprets this as insufficient pressure or sensor failure. The circuit is tested for signal integrity during key-on and operating conditions.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Sensor Voltage Output | 0.5V to 4.5V | Below 0.5V (open circuit or low signal) |
| Fluid Pressure Reading | 20-100+ PSI depending on gear | Sensor reads critically low or no signal |
Code P0842 is a moderate fault. You can generally drive to a workshop, but avoid long trips or high-load driving (motorway, uphill towing) until it is diagnosed. If the code keeps returning after clearing, or if you notice the symptoms listed above worsening, do not delay professional diagnosis. Many moderate codes have multiple possible root causes — a mechanic with live OBD data can identify the exact fault more efficiently than part-by-part trial and error.
Once the fault is repaired, P0842 can be cleared using any OBD-II scanner. Connect the scanner, navigate to "Clear Codes" or "Erase DTCs," and confirm. The check engine light turns off immediately.
The code will return if the root cause was not actually fixed. The ECM re-detects the fault within 1–3 drive cycles and sets the code again.