Coolant Leak from EGR Cooler Fitting with Motor Block

[tariqzai]

Hi there, I have an issue that has already cost me thousands of buck but still no fix. I will really appreciate your expert opinion, it's a freightliner Cascadia with ISX CM 871, 500hp. a month a go Coolant started leaking from the Fitting that hooks up EGR cooler coolant line to the Engine Block on lower back part of passenger side of engine (see pic). Mechanic took the lower fitting from engine block and replaced O rings but did't fix the problem, he then replaced the EGR cooler, it did work for about 10k miles but started leaking again, 2-3 gallons a day now. What should I do now, get O rings again, new fitting that mounts to the block or new cooler? please advise.