P249D: Closed Loop Reductant Injection Control At Limit Flow Too Low

What it means

With reductant injection running closed-loop the controller drives the dosing valve to maximum opening but cannot reach the required AdBlue flow.

At a glance

• Usually lights the check-engine light
• Affects emissions; the car can fail an emissions test

How it works

The reductant dosing system meters AdBlue/DEF into the exhaust so the SCR catalyst can reduce NOx emissions. During closed-loop dosing, the controller compares requested flow with measured flow and commands the dosing valve further open; P249D sets when the valve reaches its opening limit but flow remains below target.

Common causes

Most likely first.

1. Low reductant pressure from a weak pump or restriction
2. Blocked or restricted dosing injector
3. Leak in the reductant line
4. Faulty pressure or flow sensor

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. Check freeze-frame data and live reductant pressure and flow during a dosing event to confirm that the low-flow condition is repeatable.
2. Check AdBlue/DEF level and inspect the tank, pump area, lines, and connections for leaks or visible damage that could reduce pressure.
3. Inspect the dosing injector for deposits or crystallized reductant that could obstruct its outlet.
4. Check the reductant lines and filters, where fitted, for kinks, blockage, or restrictions that could limit delivery.
5. Test pump output and compare pressure readings with the manufacturer's specification to find a weak pump or pressure loss.
6. Check the pressure and flow sensor readings and their wiring against the manufacturer's diagnostic procedure before replacing components.

Common fixes

• Top up with the correct AdBlue/DEF if the tank is low.
• Repair a leaking or damaged reductant line or connection.
• Clean or replace a restricted dosing injector using the vehicle's service procedure.
• Clear a blocked reductant line or filter.
• Replace a pump or pressure/flow sensor only after testing confirms it is faulty.

Can I keep driving?

Limit driving and arrange diagnosis soon; dosing below target can raise NOx emissions and trigger an AdBlue warning or eventual no-start countdown. Avoid ignoring that countdown, since the vehicle may refuse to restart.