P23C4: Engine Coolant Bypass Valve C Position Sensor Circuit Low

P23C4 says the angle reported by the bypass valve C position sensor has dropped below anything the sensor can produce in normal use, which GM traces to the signal line touching ground. GM's 2025 tables use the code on both its 3.0L Duramax diesel (25OBDG04B) and a gasoline family (25OBDG03B): the SENT sensor value must fall under 27 or under 50 counts respectively, for 4 seconds within a 5-second window, while SENT communication itself is healthy and the engine is not cranking. It is a Type B fault, so the lamp comes on after the second failing trip. With the feedback gone, the valve can no longer be positioned reliably and warm-up or cooling may suffer. The opposite case is P23C5; a wider position error is P26BB.

Medium severity. Can I keep driving?Watch the temperature gauge. If it climbs or a warning appears, stop the car in a safe place and turn off the engine: overheating can cause serious damage.

Symptoms

Engine warning lamp after two drives
Bypass valve C position shown as implausibly low in live data
Slow warm-up or unusual engine temperature

Most likely causes

Position sensor signal wire chafed against ground
Coolant or water in the valve connector bridging the signal to ground
Defective position sensor inside the valve actuator
Missing sensor supply so the output stays at its lowest value

What to check, in order

Start with the simplest. You can tick off each step.

With the engine cold, check the coolant level; never do this when the system is hot
With the connector unplugged, check whether the signal wire is making contact with ground.
Check the actual temperature in live data
Make sure the fan comes on when it should
Check the thermostat and connector
Measure the sensor and replace it if its readings do not make sense

Good to know

Repair difficulty: medium
It can trigger limp mode.
It affects emissions.

What each character of P23C4 means

P: Powertrain: engine and transmission
2: Generic code: same on every make
3: Ignition and misfires
C4: Specific fault in that system

Frequently asked questions

Can I keep driving with code P23C4?

Watch the temperature gauge. If it climbs or a warning appears, stop the car in a safe place and turn off the engine: overheating can cause serious damage.

What causes code P23C4?

The most common causes are: position sensor signal wire chafed against ground; coolant or water in the valve connector bridging the signal to ground; defective position sensor inside the valve actuator.

Related codes

Sources

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.