If the tech man does the job exactly as it is suppose to be done it will be accurate within 2 CU.IN. The engine must have all the spark plugs out and the rockers removed from the cylinder in question. the P&G tube must be clean and dry,the engine is spun over on the starter and a reading taken.an accurate thermometer needs to be placed in the cylinder and the reading from the thermometer and the P&G then is adjusted by the temperature correction chart. Bill