An internal mode switch (often referred to as the park/neutral switch or transmission range selector) sends voltage signals to the TCM to let it know what gear has been selected by the driver. A P1821 DTC indicates that the voltage coming from circuit ‘B’ is low.
How Serious is the Code?
The P1821 diagnostic trouble code can be quite serious, because the TCM can respond by increasing the line pressure to the point that the clutches and other internal components become damaged.
Symptoms of a P1821 include:
- Harsh gear changes
- Transmission won’t shift into the right gear
- PRNDL display stops working
- Limited torque converter operation
- Limp mode
- Faulty internal mode switch
- Shift cable damage or it’s out of adjustment
How to Diagnose the Code?
To determine the cause of a P1821 DTC, a mechanic should inspect the shift cable and the internal mode switch to see if anything needs to be adjusted. If no issues are found, the internal mode switch has most likely failed.
What Repairs Will Fix P1821?
- Adjusting the shift cable
- Replacing the internal mode switch
P1819 – Internal Mode Switch – No Start\Wrong Range
P1820 – Internal Mode Switch Circuit A 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