P/ES Circuit Out Of Self Test Range
Your engine's transmission pressure control circuit is reading outside its expected range during self-test, like a thermostat that's not responding within normal limits. The ECU can't verify the solenoid or sensor is working correctly.
The ECU monitors the Pressure/EPC (Electronic Pressure Control) solenoid circuit during self-test startup routines. It measures voltage and current flow to verify the solenoid responds to commands and produces expected pressure feedback within calibrated thresholds.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| EPC Solenoid Voltage | 0V (off) to 12V (on) with proper current draw | Voltage outside expected range or no response during self-test |
| Transmission Pressure Feedback | 0.5-4.5V proportional to gear selection | Voltage stuck high, low, or not varying with solenoid commands |
Code P1787 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, P1787 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.