Detecting cracks in teeth is a clinical challenge. Patients may complain of diffuse pain on chewing, pain, at times, on temperature change and pain that occurs episodically. Common diagnostic tools such as radiographs and visual examination may not detect cracks. This case report shows how PTR-LUM in The Canary System can detect cracks in teeth not seen with other devices. In this clinical situation, the crack involved a portion of the mesial and distal surfaces of a mandibular second molar.