P20D1: Exhaust Aftertreatment Fuel Injector B Control Circuit/Open

Injector B is a second exhaust fuel doser, found only on layouts with two dosing points, for example twin exhaust paths. This code reports a broken control line or open coil on that second unit. No manufacturer documents P20D1 publicly; the logic would mirror injector A, where GM's diesel tables look for an off-state output voltage stuck in a mid band (P20CB, Type B). CARB wants every electrical part of an in-exhaust injection system covered by the comprehensive component checks. Most single-doser engines never set it, and many car diesels heat the filter by late in-cylinder injection instead of a doser at all.

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
Regeneration incomplete on the affected exhaust path

Most likely causes

Loose or corroded connector at doser B
Broken wire to doser B
Open solenoid winding in doser B

What to check, in order

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

Check the connector and wiring of the injector indicated
Check the continuity of the wire between the component and the control module.
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 P20D1 means

P: Powertrain: engine and transmission
2: Generic code: same on every make
0: Subsystem depends on the code range
D1: Specific fault in that system

Frequently asked questions

Can I keep driving with code P20D1?

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 P20D1?

The most common causes are: loose or corroded connector at doser B; broken wire to doser B; open solenoid winding in doser B.

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.