Exhaust treatment system with urea temperature rationality diagnostic

1. A method for monitoring an exhaust after-treatment system that doses an exhaust treatment fluid held from a tank into an exhaust stream, the method comprising:

determining a first temperature of the exhaust treatment fluid in the tank using a temperature sensor;

if the first temperature of the exhaust treatment fluid is less than a predetermined temperature, activating a heater to increase the first temperature of the exhaust treatment fluid;

monitoring the increasing first temperature of exhaust treatment fluid relative to the predetermined temperature;

determining when a phase change occurs based on the temperature of the exhaust treatment fluid reaching a plateau for a predetermined amount of time while the exhaust treatment fluid is being heated;

detecting a second temperature when the phase change of the exhaust treatment fluid occurs; and

comparing the detected second temperature to the predetermined temperature.

2. The method of claim 1 , further comprising determining whether the temperature sensor is rational based on the detected second temperature.

3. The method of claim 2 , wherein if the detected second temperature is greater than the predetermined temperature, the temperature sensor is irrational.

4. The method of claim 2 , wherein if the detected second temperature is less than the predetermined temperature, the temperature sensor is irrational.

5. The method of claim 1 , further comprising determining whether the exhaust treatment fluid is of sufficient quality based on the comparison of the detected second temperature and the predetermined temperature.

6. The method of claim 5 , wherein if the detected second temperature is greater than the predetermined temperature, the exhaust treatment fluid is not of sufficient quality.

7. The method of claim 5 , wherein if the detected second temperature is less than the predetermined temperature, the exhaust treatment fluid is not of sufficient quality.

8. The method of claim 1 , wherein the predetermined amount of time at the plateau temperature ranges between 5 and 10 minutes.

9. The method of claim 1 , wherein the predetermined temperature is a phase change temperature of the exhaust treatment fluid.

10. The method of claim 1 , further comprising determining that the temperature sensor is irrational if no second temperature is detected.

11. An exhaust after-treatment system for treating an exhaust produced by an engine, comprising:

an exhaust treatment component;

an injector positioned upstream from the exhaust treatment component for injecting an exhaust treatment fluid into the exhaust;

an exhaust treatment fluid tank that stores and provides the injector with the exhaust treatment fluid;

a temperature sensor positioned in the tank for detecting a temperature of the exhaust treatment fluid;

a fluid heater positioned in the tank for increasing a temperature of the exhaust treatment fluid; and

a controller for controlling each of the injector, temperature sensor, and fluid heater,

wherein the temperature sensor determines a first temperature of the exhaust treatment fluid in the tank and communicates the first temperature to the controller, and if the first temperature of the exhaust treatment fluid is less than a predetermined temperature, the controller activates the heater to increase the first temperature of the exhaust treatment fluid;

the controller monitors the increasing first temperature of exhaust treatment fluid detected by the temperature sensor relative to the predetermined temperature, and determines whether a temperature plateau is maintained for a predetermined amount of time while the exhaust treatment fluid is being heated to indicate a phase change, the controller detects a second temperature when the phase change of the exhaust treatment fluid occurs; and

the controller compares the detected second temperature to the predetermined temperature.

12. The system of claim 11 , wherein the controller determines whether the temperature sensor is rational based on the detected second temperature.

13. The system of claim 12 , wherein if the detected second temperature is greater than the predetermined temperature, the controller determines that temperature sensor is irrational.

14. The system of claim 12 , wherein if the detected second temperature is less than the predetermined temperature, the controller determines that temperature sensor is irrational.

15. The system of claim 11 , wherein the controller determines whether the exhaust treatment fluid is of sufficient quality based on the detected second temperature.

16. The system of claim 15 , wherein if the detected second temperature is greater than the predetermined temperature, the controller determines that the exhaust treatment fluid is not of sufficient quality.

17. The system of claim 15 , wherein if the detected second temperature is less than the predetermined temperature, the controller determines that the exhaust treatment fluid is not of sufficient quality.

18. The system of claim 11 , wherein the exhaust treatment fluid is a urea exhaust treatment fluid.

19. The system of claim 11 , wherein the predetermined temperature is a phase change temperature of the exhaust treatment fluid, the predetermined temperature being stored in the controller.

20. The system of claim 11 , wherein if a temperature plateau is not reached for the predetermined amount of time, the controller determines that the exhaust treatment fluid is not of sufficient quality.