When a DEF heater fails, it prevents the diesel engine's exhaust aftertreatment system from functioning properly in low-temperature environments. The most immediate consequence is excessive NOx (nitrogen oxide) emissions, which can subsequently trigger a range of operational issues for the vehicle.

Specific symptoms and consequences include:

**Difficulty with Cold Starts or Reduced Emission System Efficiency**
DEF (urea solution) is prone to crystallization at low temperatures; the function of the heater is to ensure the urea solution remains in a liquid state and is injected correctly. If the heater malfunctions, the urea cannot be effectively atomized, rendering the SCR (Selective Catalytic Reduction) system unable to efficiently convert harmful gases within the exhaust.

**Activation of Vehicle Power Limitation or "Limp Mode" Protection**
Most modern diesel vehicles, upon detecting an anomaly in the DEF system (such as a heater failure), will utilize the ECU to limit engine power. This measure compels the owner to seek repairs as soon as possible, thereby preventing continued high-level pollutant emissions.

**Illumination of the DEF Fault Light on the Dashboard**
The vehicle will typically generate multiple related diagnostic trouble codes simultaneously—for instance, "DEF heater circuit malfunction," or accompanying alerts such as "Low SCR system efficiency." Professional diagnostic equipment is required to retrieve and interpret this specific information.

**Potential Inability to Start the Vehicle in Extreme Cold**
In cold climates, if the DEF heater fails completely and the DEF solution freezes, the system may classify this as an unrecoverable fault. Certain vehicle models will subsequently prevent the engine from starting until the underlying issue has been resolved.

It is strongly recommended that you contact professional repair personnel as soon as possible to conduct a thorough inspection. Particular attention should be paid to verifying the proper functioning of the heater circuit, relays, and temperature sensors. If this issue is neglected over an extended period, it will not only compromise vehicle performance but may also lead to clogging or damage of the catalytic converter, resulting in significantly higher repair costs.