P233B: Fuel Pressure Regulator C Control Circuit Low

P233B points to the ground side of fuel pressure regulator C: the line between valve and low-side driver has contact with ground. In GM's 2025 diesel table 25OBDG04B, regulator C is the second fuel metering unit valve, and the ECM checks this line while the driver is switched off. If the voltage there is too low, meaning less than about 0.5 Ohm towards ground, the sample counts as failed; sampling runs at 100 ms with more than 11 V supply and the engine not cranking, and the code is Type A with the MIL in the first trip. A grounded low-side line can energise the valve permanently, so pump delivery no longer follows the command. GM bulletin 21-NA-149 names P233B for the 3.0L LM2 diesel after harness chafing. The open-circuit test is P233A, the opposite short P233C.

Medium severity. Can I keep driving?Avoid long trips and hard acceleration. If you smell fuel or the car stalls, do not continue.

Symptoms

Check engine light on immediately
Rail pressure too high or unstable, rough running
Reduced engine power possible

Most likely causes

Insulation of the control wire worn through against engine or bracket
Moisture or corrosion bridging pins in the valve connector
Metering valve winding shorted to its housing
Low-side driver in the engine control module damaged

What to check, in order

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

Check the pump's fuse, relay, and connector
With the connector unplugged, check whether the signal wire is making contact with ground.
Replace the filter if it is due for maintenance
Compare commanded and actual pressure in live data
Measure the pressure with a suitable gauge
Check the sensor and regulator before replacing the pump

Good to know

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

What each character of P233B means

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

Frequently asked questions

Can I keep driving with code P233B?

Avoid long trips and hard acceleration. If you smell fuel or the car stalls, do not continue.

What causes code P233B?

The most common causes are: insulation of the control wire worn through against engine or bracket; moisture or corrosion bridging pins in the valve connector; metering valve winding shorted to its housing.

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.