P2D1D: Reductant System Leak Detected – Small Leak

What it means

The SCR module detects a small leak in the reductant system based on pressure decay rate after dosing pause. Slow loss of DEF/AdBlue.

At a glance

• Usually lights the check-engine light
• Affects emissions; the car can fail an emissions test
• The engine or gearbox may go into limp mode

How it works

The reductant system stores and delivers DEF or AdBlue through lines and fittings to the SCR injector, which helps reduce nitrogen oxide emissions. After dosing pauses, the SCR module monitors pressure loss; this code sets when pressure decays at a rate consistent with a small leak, such as at a connection or seal.

Common causes

Most likely first.

1. Fitting or connection loose
2. Seal aged at injector or pump

Symptoms

• Check engine light or NOx warning on
• AdBlue/DEF level or quality warning
• Possible no-start countdown after ignore period
• Failed emissions inspection (NOx)

How to diagnose it

1. Record freeze-frame data and check scan-tool data for reductant pressure decay and any SCR warnings or countdown status.
2. With the system safe to inspect, look for DEF or AdBlue crystals, dampness, or residue around accessible lines, fittings, the pump, and injector.
3. Check accessible reductant connections and seals for looseness or visible damage without opening a pressurized system.
4. Use the manufacturer's leak test to verify pressure decay and isolate whether the loss is at a line, fitting, pump, or injector seal.
5. If no external leak is found, have the SCR system's pressure-holding components checked using the manufacturer's diagnostic procedure.

Common fixes

• Tighten a loose reductant fitting if the manufacturer's procedure permits.
• Replace a damaged or aged seal at the leaking pump or injector connection.
• Replace a leaking reductant line or fitting.
• Repair or replace the pump or injector if the specified leak test locates the pressure loss there.

Can I keep driving?

You can usually drive cautiously if there is no active fluid spray or other warning requiring a stop, but arrange repair promptly and heed any DEF or AdBlue countdown. Ignoring a leak can lead to emissions faults and, on some vehicles, a no-start restriction.