Gruesome photos of Celeste Rivas' dismembered and decomposing remains were shown during D4vd's preliminary hearing ... but testimony about the unimaginable smell was just as powerful.

Dr. Grant Ho of the Los Angeles County Department of Medical Examiner told the judge the odor emanating from the teen's remains was so overwhelming they could actually be smelled down the hall before being brought into the examination room for the autopsy.

The horrifying detail is compelling because it was the smell of her decomposing remains that resulted in the case being opened in the first place ... the odor leading to the grisly discovery last September inside a Hollywood tow yard.

Prosecutors say the Tesla where Celeste's remains were found was loaded with air fresheners ... 11 in total.

After five days of detailed and graphic testimony from 12 different witnesses during D4vd's preliminary hearing ... the judge ordered the murder case to proceed to trial, which must begin within 90 days.

D4vd is facing a murder charge with special circumstances, which makes him eligible for the death penalty ... though prosecutors haven't made a decision either way about seeking capital punishment.

He is also charged with continuous sexual abuse of a child under the age of 14, and unlawful mutilation of human remains. He has pleaded not guilty.