When a planet is combust it means it’s conjunct or very close to the Sun. The planet gets ‘burnt away’, it becomes invisible and loses strength. It becomes difficult for the planet to provide results, it could take longer or it may not be very favorable, significant. Results could come with difficulty most of the time.

Planets combust

Venus combust = Can give relationship issues, losing control easily over finances and love, not being careful enough
Mercury combust = Intellectual struggles in which the ego could overpower
Mars combust = Can make aggressive and egocentric, strong-willed and little patience. Competitive
Jupiter combust = Big ego, ego obsessive issues, loss of belief or lack of knowledge
Saturn combust = Sense of responsibility is weakened, issues with being grounded and the ego

Degree (orb) in which a planet becomes combust
Venus = 10 degrees
Mercury = 14 degrees / 12 degrees in retrograde
Mars = 17 degrees
Jupiter = 11 degrees
Saturn = 16 degrees

House rulers combust

1st = Health issues, weak immune system, general struggling in life
2nd = Issues with family ties, financial issues, problems with food, bad speech
3rd = Issues with siblings, miscommunication, lack of courage, issues with short travels
4th = Issues with mother, no own house
5th = Issues with children, pregnancy, issues in love
6th = Issues with resistance to disease, routines, loss of legal cases, job issues
7th = Issues in relationships, marriage, problematic business relationships, weak stomach
8th = Trust issues, difficulty with sharing and partnerships, accident prone
9th = Believe system issues, lack of fortune, loss of father
10th = Issues with authority and finding purpose, issues with father
11th = Issues in friendships, difficulty in gaining fortune, money struggles
12th = Feeling isolated, issues with loss, problems with sleep or sexual pleasures

House of combustion

Where ever the combust planet(s) fall in a horoscope, can tell more about it’s influence on the native’s life.
In astrology there are 3 malefic houses : 6, 8, 12
If a combust aspect takes place in any of these houses, it creates more difficulty.
A remedy whenever a combust planet is located in one of the malefic houses, is relocation.
By moving the aspect to a more benefic house, the issues could be less damaging, however the aspect cannot be changed, and there is always a reason why you were born with it in the first place, to learn to master it, to balance, and to work on your sense of awareness depending on which planet is combust.
Learn more about relocation :
Local space chart

Dignity of combustion

A well dignified planet that’s combust, still gives difficulties or issues, however the planet gains more strength and awareness. A well dignified planets means that it’s placed in domicile or in exaltation. Such as Mars in Aries is in domicile, Moon in Taurus is exalted.
You can find more info about the planetary dignities here .