In order to answer the said question we will first need to clearly differentiate between Investigation & Inquiry. Investigation is done by the police officer or any person authorised, which involves collecting of evidences for the purpose of inquiry, where the persons are examined & interrogated. Inquiry on the other hand is undertaken by the magistrate or court in order to answer a said question judicially under the code. Another term that is called in question is Inquest. Inquest can be done by the police or the magistrate especially for ascertainment of the cause of death in cases of suicide, unnatural death, death caused in police custody (exclusively taken up by the Magistrate) etc. After we get a hang of the three terms we will be able to distinguish between the two sets of sections present in the code.
Section 154, 156 & 157 of the code deal exclusively with the power of the police to get an information of a cognizable offence, investigate that offence & follow the procedure for that investigation mentioned is section 157. While, if we study section 174 & 176 of the code, the main objective is merely to ascertain whether a person has died under suspicious circumstances or unnatural death and if so what is the apparent cause of death. After which an inquest report has to be made describing the same. To specifically draw a difference between section 154 & 174, section 174 is not as detailed as section 154. Inquest report does not require the details of the accused, or details of the incident, which are the main ingredients of an FIR & investigation report. After making of the inquest report the police officer shall hold an investigation. But the main objective of section 174 & 176 remains mere ascertainment of cause of death which is suspicious or unnatural. Section 176 deals majorly with Custodial deaths & rapes, on which an inquiry is held by the magistrate. This inquiry is other than or in addition to a police investigation. The main aim being that in cases of custodial deaths and rape, police officers & their investigations are not always to be trusted.
Thus section 154, 156 & 157 deal with power of police & section 174 & 176 gives power mainly to the magistrate to keep a check on the investigating powers of the police & not solely rely on the investigation report of the police, especially in cases of unnatural deaths, custodial deaths etc.