What are the symptoms of CABG failure?

Same as CAD. CABG failure either means the grafts clotted off or else the grafts were not placed in the best area to bypass all the blockages, especially in patients with diffuse blockages where there may not have been a good place no matter what ... Symptoms are return of the angina, decreased exercise tolerance, fatigue, shortness of breath, lightheadedness, nausea, sweating, etc.
CABG. These are unusual, by failure you mean failure of the grafts, so signs of graft failure would be an inability to come off bypass pump, heart failure, myocardial infarction, or arrhythmia. Any of these are serious and can be fatal. If the failure occurs early after surgery then the patient could be brought back to the or, if it occurs later then the patient may be brought to the cath lab.