What does it mean when the Transit DEF light is on?
3 Answers
The Transit DEF light being on indicates that the car's auxiliary water tank is low on water. If the small odometer on the car's dashboard displays DEF, it suggests an internal processor fault in the car's dashboard. If the auxiliary water tank is low on water, it needs to be refilled immediately to resolve the issue; if the problem lies with the internal processor of the car's dashboard, it should be handled by professional repair personnel. The auxiliary water tank is usually located under the driver's seat. Remove the seat cushion and then the square metal plate to access it. Reasons for the DEF light being on: The main function of the car's auxiliary water tank is to allow coolant to flow into it through the radiator cap when the engine temperature is high and the liquid expands. When the temperature drops and the cooling system pressure becomes negative, the system draws the coolant back from the auxiliary water tank into the cooling system via the radiator cap. Additionally, the auxiliary water tank has corresponding markings indicating the liquid level, allowing for easy visual inspection to check if the water level is sufficient. The auxiliary water tank is used for cooling, and for car owners, it requires periodic water refills. The auxiliary water tank serves as an auxiliary to the main radiator and is an essential component of the cooling system.
Last time I drove the Transit for delivery, the DEF light also came on, which really freaked me out. This light specifically monitors the emission system of diesel vehicles, especially the particulate filter in the exhaust pipe. Simply put, it means carbon particles in the exhaust have clogged the filter, and the system can't burn off the carbon buildup, triggering the light. If this happens, don't panic right away. Find a clear stretch of highway, maintain a speed of 80 km/h for about 20 minutes to let the engine heat up and burn off the carbon deposits. If the light stays on, you'll need to check the urea injection system—see if the nozzle is clogged or if the urea fluid level is sufficient. If all else fails, connect a diagnostic tool to check the fault codes; sometimes the differential pressure sensor might be malfunctioning and giving false readings. Never ignore it—your vehicle will definitely fail the emissions test during inspection, and in severe cases, the engine's power might even drop.
The DEF light usually indicates an alarm in the diesel vehicle's exhaust treatment system, which is particularly common in Transit models. There are three typical scenarios: either the DPF filter is clogged with excessive carbon buildup, causing restricted exhaust flow and a sharp increase in fuel consumption; or the urea fluid level is low, requiring immediate refill with specialized urea; another possibility is a false sensor alarm, especially after driving through muddy or watery roads. When dealing with it myself, I first check the dashboard alerts—if the urea indicator is also lit, I head straight to the urea tank. If only the DEF light is on, I try revving the engine three times at idle before restarting, as the system can sometimes recover automatically. If the light persists, I recommend bringing a bottle of urea and visiting a repair shop to read the fault codes, as improper repairs could cause major damage to the SCR system.
