Wierd smell
 

My girlfriend drove her mazda 3 to work this morning and everything seemed fine, after about 15 min she was told that she did not have to work today so she left. upon starting her vehicle and backing up her engine light came on. after about 2min of driving she noticed a burning smell. she proceeded to drive to my house about 20min away and noted that the smell was stronger when she slowed down or downshifted. we plan on taking it in to get checked out but i am just wondering if anyone here might have an idea of what the problem may be. oh also some things to consider, its been pretty cold lately (45) and she has been running really heavy oil.. 20/50 i think. and she also has a cold so she cant really smell things too well anyway.