P1720 (C8) - Line Pressure Error

Name of Code:
P1720 (C8) - Line Pressure Error

When Monitored:
Continuously with engine running.

Set Condition:
The TCM has an internal error counter for each DTC above. When any of the counters reaches a preset threshold, the appropriate code is set.

Theory of Operation:
Line pressured is controlled by the TCM. It is measured by the Line Pressure Sensor (LPS) and regulation is achieved by changing the duty cycle of the Pressure Control Solenoid (PCS). (0% duty cycle = solenoid off = max line pressure, 100% duty cycle = solenoid on = min line pressure). The TCM calculates the desired line pressure based on inputs from the transmission and engine. The TCM calculates torque input to the transmission and uses it as the primary input to the desired line pressure calculation. This is called Torque Based Line Pressure. In addition, the line pressure is set to a preset level (100 or 120 PSI) during shifts and in Park and Neutral to ensure consistent shift quality. The desired line pressure is continuously being compared to the actual line pressure. If the actual line pressure is consistently higher than the target, the line pressure high DTC P1724 (CB) will be set. If the actual line pressure is consistently lower than the target, the line pressure low DTC P1722 (C9) will be set. If it is consistently out of range, but not consistently high or low, it is considered out of range and the line pressure error DTC P1720 (C8) will be set.

Transmission Effects:
The MIL will illuminate. The Transmission system will default to open loop line pressure control resulting in a fixed PCS duty cycle. This duty cycle will change slightly depending on temperature and current gear.

Possible Causes:
- Sticking Regulator Valve
- PCS
- Line Pressure Sensor.
- Oil pump
- Plugged Filter
- 5V supply circuit to Line Pressure Sensor open (Sensor reading will read a constant value between 85 and 95 PSI)
- TCM