General perception of Pitridosh is that forefathers of the native are unhappy and cursing the native. Actually, it is other way round, that FOREFATHERS ARE THEMSELVES CURSED.
Horoscope of a person may reflect many doshas(afflictions) viz. Kalsarp Dosh or Pitridosh etc. The question that arises is “ What bad karma a person could do at birth itself to get cursed by ancestors?”
In most of the cases, a person born in a family tree is none other than one of his own ancestors. According to theory of Karmik debt, most of the people are born in the same family lineage with same friends and enemies in so many of their lives until their karmic debts, duties and rewards gets all cleared from that particular group of people/souls.
So, a Pitradosh is not a curse of ancestors but it is a Karmic debt of the ancestors which the person having Pitradosh in his horoscope is bound to pay.
As we know a number of disease are genetic in nature and so is this dosh, passed on from parents to children. Isn’t the same true in legal system also? Since we enjoy the wealth ,name and fame earned by our ancestors, we are equally liable to suffer their misdeeds also. So, Pitra dosh is not curse of ancestors but it is a debt formed as e result of bad deeds done by the ancestors , to be paid by afflicted native by suffering punishment or by doing some good Karmic deeds.
King Bhagirath had to do Tapasya to bring Ganga to Earth to get rid of this dosh and subsequently moksha of his ancestor King Sagar.
पितृ दोष के कारण –
पितृ दोष उत्पन्न होने के अनेक कारण हो सकते है . जैसे:- परिवार में अकाल मृ्त्यु हुई हों, परिवार में इस प्रकार की घटनाएं जब एक से अधिक बार हुई हों . या फिर पितरों का विधी विधान से श्राद्ध न किया जाता हों, या धर्म कार्यो में पितरों को याद न किया जाता हो, परिवार में धार्मिक क्रियाएं सम्पन्न न होती हो, धर्म के विपरीत परिवार में आचरण हो रहा हो, परिवार के किसी सदस्य के द्वारा गौ हत्या हो जाने पर, या फिर भ्रूण हत्या होने पर भी पितृ दोष व्यक्ति कि कुण्डली में प्रकट होता है .
The following combinations indicate Pitridosh in horoscope:
1. Sun is afflicted by Rahu/Ketu by conjunction or aspect. Sun-Rahu conjunction is worst and more so in 6/8/12 houses.
2. Sun afflicted by Saturn or any other malefic(natural or functional) also causes this dosh.
3. 9H/9L afflicted by Rahu-Ketu or a malefic also causes this dosh.
4. Moon-Ketu conjunction and implied aspect of Rahu constitutes Matru dosh.
5. Luminaries as AtmaKarak associated with 8L.
The symtoms/obstacles caused by this dosh are as below:
a)Difficulties in conceiving child or frequent miscarriages.
b)Bearing mentally challenged children or death of children.
c)Addictions, obstacles in marriage and in general , loss of job, going blank in exams.

Pitridosh as per Lal Kitab:

घर नौवे हो कोई ग्रह बैठा, बुध बैठा कोई साथी हो
ऋण पितृ उस घर से होगा असर ग्रह सब निष्फल हो
साथी ग्रह जब जड कोई काटे दृष्टि मगर वो छुपता हो
पाँच बारह दौ नौ कोई मंदे, ऋण पितृ बन जाता हो
I have tried to interpret with my limited knowledge as below:

1. If there is a planet in 9H and Mercury is posited in the Rasi of that planet, then it will be a case of PitriRin, for example if JUP. is in 9H and Mer is in Saggitarius or Pisces then it is an indication of dosh.
2. When enemy planets occupy a Rasi and that Rasi is not aspected by the Rasi Lord, for example, if enemy planet of Sat like Sun Moon or Mars are in Capricorn or Aquarius and Sat doesn’t have an aspect on its Rasis then it will further aggravate the condition of dosh.
3. If there is an enemy planet in the Pucca Ghar of a planet. To give an example, Pucca Ghar of Jup are 2/5/9/12 and if Ven or Mer are in these houses then it will be a kundali of pitriRin.
If all the three conditions mentioned above are fulfilled then the horoscope is said to be afflicted with pitridosh , otherwise not.

$ General(Vedic) remedies of Pitridosh

a) Feed crows and fish with rice mixed with Ghee and made into balls on every Saturday.
b) Give Daan on Amavasya.
c) On Somavati Amavasya, go to Peepal tree and offer a Janeu(thread) to tree and Vishnu each and offer Water enchanting Om Namo Bhagvate Vasudevaye !
d) Collect equal money from family members and donate to a temple.

Remedies as per Lal Kitab:

a) Durga Pujan and offer sweets to the girls and obtain their blessings.
b) Service of elders.
c) Install a hand pump in ancestral house.
d) Feed the blind.
e) Feed the monkeys.
f) Do Kanyadaan.
g) Wear white cap or turban.
h) Gold or Golden Ornaments be kept in yellow cloth.
i) Silver brick be buried in foundation.
j) Apply Tilak of Kesar or haldi.
k) Maintain good moral character.

The best thing to appease Pitra Devta is to perform Shraadh in PitriPaksh,   Please excuse me for any inadvertent mistake.
Om Hari !

compiled by Ashutosh