The man was picked up by police on Tuesday in the car repair workshop where he works and remains in custody.
His former partner, a 41-year old woman known by the initials R.P.M., received a number of superficial scalp injuries when the device detonated on Monday morning. She was taken to Vinapoló hospital in the autonomous Valencia region but the relatively low amount of shrapnel involved meant that she could be treated and quickly discharged.
Emergency services were originally told that the woman's car airbag had exploded, causing burns but experts from the police's TEDAX bomb squad discovered evidence of what they described as a "homemade" but "low intensity" bomb, according to Spanish daily El País.
The car has been impounded as evidence and will be studied for further details of the device's construction.