hey it looks like you used the wrong type of tester for this job. i know you are using what you have available but this didnt work. you are using, i guess, an air gas tester not meant to get wet.
i saw my lead mechanic at work take a sample of coolant from the reservoir and it is measured in a glass beaker, then i think he had some dye drops that go into the coolant sample to determine if there is exhaust gas inside your coolant. this is a proper way to perform the "head gasket failure diagnosis"
if you have loss of power complaint as well then i would guess 2 cylinders are low on compression due to the head gasket leaking your compression into the cooling system.
i have a EASY way to check as well. while the car is cold, check the pressure on the radiator hose by squeeze with your hand. then start the car and let it run for 1 min, then shut it off. now check if that upper radiator hose has pressure that was not there 1 min ago. the water in that system will only make 5-10 psi ONCE ITS HOT, so in 1 min that coolant didnt get hot, so if there is any pressure it came from your cylinder compression.
i saw my lead mechanic at work take a sample of coolant from the reservoir and it is measured in a glass beaker, then i think he had some dye drops that go into the coolant sample to determine if there is exhaust gas inside your coolant. this is a proper way to perform the "head gasket failure diagnosis"
if you have loss of power complaint as well then i would guess 2 cylinders are low on compression due to the head gasket leaking your compression into the cooling system.
i have a EASY way to check as well. while the car is cold, check the pressure on the radiator hose by squeeze with your hand. then start the car and let it run for 1 min, then shut it off. now check if that upper radiator hose has pressure that was not there 1 min ago. the water in that system will only make 5-10 psi ONCE ITS HOT, so in 1 min that coolant didnt get hot, so if there is any pressure it came from your cylinder compression.