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DEF warning light on

DEF quality, dosing and NOx faults traced properly — because a DEF lamp ignored becomes a speed derate, and a derated truck cannot work.

DEF & SCR SCR
2.0 h
Diagnostic scan
Same day
Most DEF faults
48 hr
Typical booked slot
4
OEM scan platforms

Do not top up with questionable fluid before you come in — we test what is in the tank as part of the diagnosis.

Likely causes

What sets off a DEF lamp.

The lamp means the system does not trust something. Sorting which something is the whole job.

Contaminated or off-spec fluidWater, diesel or the wrong concentration. We test the fluid in the tank before touching a sensor.
Failed quality sensorGood fluid, bad reading. Verified against a tested sample rather than swapped on a guess.
Dosing unit or injector blockedCrystallised urea blocks the nozzle. Cleaned or replaced with flow verified after.
NOx sensor driftInlet or outlet sensor reading outside tolerance triggers SCR efficiency codes.
Frozen or heated line faultWinter faults from a failed heater circuit or pump. Common in Ontario cold snaps.
Exhaust leak in the SCR pathLeaks skew NOx readings and set efficiency faults that look like sensor failure.
Our path

Lamp to cleared, properly.

01

Test the fluid

Concentration and contamination checked before any part is condemned.

02

Scan and read live

Dosing commands, NOx values and SCR efficiency watched under load.

03

Verify the part

Sensor, pump or injector confirmed faulty — not replaced on a code alone.

04

Repair and reset

Fault cleared, derate countdown reset, unit re-scanned before it leaves.

Get it diagnosed

Get the countdown stopped.

DEF faults escalate on a timer. Send the code and your remaining distance and we will slot you accordingly.

Speed derate warning showing? (905) 677-7663
Can you delete the DEF system instead?+

No. Emissions deletes are illegal on road-registered trucks in Ontario and will fail an annual inspection. We repair the system.

Why did the light come on right after a fill?+

Most often the fluid. Contaminated or diluted DEF from a bad drum sets a quality fault almost immediately.

Will it pass an annual with the lamp on?+

An active emissions fault is a problem at inspection. Get it cleared before you book the annual, or book them together.

Send the fault

Callback within one business hour, Mon–Sat.

Goes straight to the service desk.
Related

Related work

Full diagnosticsFour OEM scan platforms, faults traced to the harness.DPF & regenFilter cleaning, forced regens, aftertreatment repair.DEF / SCRDosing, quality sensors and NOx faults sorted properly.Annual inspectionBook the MTO annual while the unit is already in the bay.