Sometimes referred to as the park/neutral switch or transmission range selector, the internal mode switch is a sliding contact switch on the valve body that tells the transmission control unit what gear you have selected. If the voltage signal coming from the C circuit is high for too long (the switch is in the open position), then a P1826 diagnostic trouble code will be stored.
How Serious is the Code?
The P1826 DTC is very serious, because the TCM will apply maximum line pressure, and that will damage the clutches, valves, seals and other vital components. The transmission may go into limp mode, but at the very least the torque converter lockup clutch will stop functioning. As will most of the solenoids and the adaptive shift program.
Symptoms of a P1826 include:
- Extremely rough gear changes
- Transmission refuses to go into the right gear
- PRNDL display stops working
- Limited torque converter operation
- Limp Mode
- Internal mode switch failure
- Shift cable damage or it’s out of adjustment
How to Diagnose the Code?
When diagnosing a P1826 OBDII diagnostic trouble code, a mechanic should first check the shift linkage going to the internal mode switch to see if it is out of adjustment. If nothing is out of adjustment, then electrical tests will confirm the internal mode switch failure.
What Repairs Will Fix P1826?
- Adjusting the shift cable
- Replacing the internal mode switch
P1819 – Internal Mode Switch – No Start\Wrong Range
P1820 – Internal Mode Switch Circuit A Low
P1821 – Internal Mode Switch Circuit B Low
P1822 – Internal Mode Switch Circuit B High
P1823 – Internal Mode Switch Circuit P Low
P1824 – Internal Mode Switch Circuit P High
P1825 – Internal Mode Switch Invalid Range