Faulty or worn internal mode switch (transmission range switch) contacts; Shift cable or linkage out of adjustment, so the switch does not match the selector position; Damaged wiring, chafing or short to ground in the P circuit between the switch and the control module; Corroded or loose connector at the transmission or control module, including water ingress; Contaminated or low transmission fluid affecting the switch or internal harness; Fault within the transmission control module.

P1823

Internal Mode Switch Circuit P Low.

Chevrolet
Powertrain
Manufacturer code
Medium severity

How this code is built

P: SystemPowertrain
1: Defined byThe manufacturer
8: AreaTransmission
23: FaultThe specific fault in that area

What it means

On Chevrolet vehicles P1823 means the transmission control module has detected a low signal in the P circuit of the internal mode switch (transmission range switch). The module compares the switch's combination of signals with the expected pattern for the selected gear position, and a low voltage or invalid state on the P input sets the code. The exact wiring and switch layout depend on the model and transmission.

Symptoms

Check engine light or transmission warning light on
Gear selector position shown incorrectly on the dash, or no display
Engine will not start in Park or Neutral, or starts in other gear positions
Harsh, delayed or incorrect shifts
Transmission stuck in one gear (limp mode)
Reverse lights not working correctly

Possible causes

Faulty or worn internal mode switch (transmission range switch) contactslikely
Shift cable or linkage out of adjustment, so the switch does not match the selector positionlikely
Damaged wiring, chafing or short to ground in the P circuit between the switch and the control modulepossible
Corroded or loose connector at the transmission or control module, including water ingresspossible
Contaminated or low transmission fluid affecting the switch or internal harnesspossible
Fault within the transmission control modulerare

How to diagnose

Read all stored and pending codes and note the freeze frame data, including gear selector position and vehicle speed
Check for other transmission, speed sensor or network codes that may point to a shared cause
Inspect the transmission connector and wiring for chafing, fluid contamination, corrosion and pin damage
Check the transmission fluid level and condition, following the procedure for the specific model
Check the shift cable and linkage adjustment so the selector position matches the position of the switch
With a scan tool, watch the live mode switch inputs while moving the selector through each position and compare them with the specification for the model
Test the P circuit for a short to ground and for correct voltage and continuity against the wiring diagram
Check the switch itself against the manufacturer's test values for that transmission

How to fix

Adjust the shift cable or linkage and set the range switch alignment to the manufacturer's procedure
Repair or replace damaged wiring and correct any short to ground in the P circuit
Clean or replace corroded or damaged connectors and terminals
Replace the internal mode switch if it fails testing
Correct the transmission fluid level or condition if it is contaminating the switch
Repair or replace the control module only after the wiring and switch are proven good

Can I keep driving?

Medium severity

The car can usually be driven short distances, but incorrect gear detection can cause no-start, poor shifting or limp mode, so get it checked soon. Stop driving if it will not select gears reliably or shifts harshly.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.