P20CD: Exhaust Aftertreatment Fuel Injector A Control Circuit Low

Here the control line of the hydrocarbon doser has a short to ground. GM's diesel tables 18OBDG04 and 25OBDG04A test the output while the driver is off: if the voltage drops below about 2 V, the line is touching ground. GM rates it Type A, which lights the MIL after one trip, and the fault only clears after a calibrated count of good tests or a control-module reset. The LML/LGH Duramax lists it as Type B. A grounded line can make the doser open whenever its supply side is switched on, so fuel may enter the exhaust uncontrolled; that is why it is treated more strictly than the open circuit (P20CB) or the short to power (P20CE).

Medium severity. Can I keep driving?Drive as little as possible and go to a repair shop. If you smell fuel or the engine is shaking badly, do not keep driving.

Symptoms

Check engine light on after a single trip
Doser disabled, no active regeneration
Possible fuel smell or smoke from the exhaust

Most likely causes

Control wire chafed through against the frame or exhaust
Water or salt in the doser connector bridging to ground
Internal short in the doser solenoid

What to check, in order

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

Check the connector and wiring of the injector indicated
With the connector unplugged, check whether the signal wire is making contact with ground.
Measure the resistance and compare it with the manufacturer's specification
Check the control signal with an oscilloscope; on direct injection or diesel engines, leave this to a repair shop because of the high voltages
On diesels, check the per-cylinder injection corrections

Good to know

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

What each character of P20CD means

P: Powertrain: engine and transmission
2: Generic code: same on every make
0: Subsystem depends on the code range
CD: Specific fault in that system

Frequently asked questions

Can I keep driving with code P20CD?

Drive as little as possible and go to a repair shop. If you smell fuel or the engine is shaking badly, do not keep driving.

What causes code P20CD?

The most common causes are: control wire chafed through against the frame or exhaust; water or salt in the doser connector bridging to ground; internal short in the doser solenoid.

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.