The internal mode switch, sometimes referred to as the park/neutral switch or transmission range selector, tells the TCM what gear has been selected based on the input voltage coming from the switch. A P1820 DTC means the signal from the ‘A’ circuit is low and the TCM cannot tell what gear has been selected.
How Serious is the Code?
A P1820 diagnostic trouble code is fairly serious due to the fact that the transmission control unit will increase the line pressure to compensate for the possibility of an incorrect gear ratio, which can cause significant damage to internal components.
Symptoms of a P1820 include:
- Unusually harsh shifting
- Gear engagement failure
- PRNDL display stops working
- Limited torque converter operation
- Limp Mode
- Faulty internal mode switch
- Shift cable damage to or out of adjustment
How to Diagnose the Code?
To diagnose a P1820 diagnostic trouble code, a technician will first need to inspect the shift cable and the internal mode switch to if anything is out of adjustment. If everything checks out, internal mode switch has failed.
What Repairs Will Fix P1820?
Adjusting the shift cable
Replacing the internal mode switch
P1819 – Internal Mode Switch – No Start\Wrong Range
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
P1826 – Internal Mode Switch Circuit C High