With the tightening requirements on engine emission and performance, pressure based combustion controls are becoming common in medium speed large bore reciprocating internal combustion engines. The accuracy of the cylinder pressure data including the raw pressure value at its corresponding crank angle, has a vital impact on engine controllability. For instance, this work shows that a 1-bar pressure offset leads to a 0.6% variation in the total heat release (THR) while the 50% heat release crank angle (CA50) can be shifted by 1.5 degrees. Similarly, with a single degree error in the crank position, the indicated mean effective pressure (IMEP) gets a 1.8 bar error. Thus, in this work the typical errors for cylinder pressure measurement are reviewed and analyzed for large bore four stroke marine and power plant production engines.The main sources of error for pressure measurement are thermal shock and installation defects. Meanwhile, calibration is carried out for ten production pressure transducers to provide a general accuracy result of the pressure transducers that are used in production engines.The main sources of error for crank angle position monitoring - when done with a flywheel-based inductive system, manufacturing tolerance, installation, and the relative displacement between the pickups and the shaft due to shaft bending, shaft longitudinal movement, torsional vibration and engine block vibration are the main sources of error.In this paper, those errors are quantified individually through simulation and their impacts on IMEP and CA50 are also presented. At last, cylinder volume deformation and its impact on combustion diagnostics are also estimated. From the result it is concluded that torsional vibration and cylinder volume deformation have the most significant effects for combustion analysis.