Editing On-board diagnostics (section)

== OBD-II diagnostic trouble codes ==

OBD-II diagnostic trouble codes ([[OBD-II PIDs|DTCs]])<ref name="obdadvisor_codes">{{Cite web |last=Miller |first=Tim |date=October 25, 2021 |title=OBD2 Codes Guides and List for Free Download |url=https://www.obdadvisor.com/most-popular-obd2-codes/ |website=OBD Advisor}}</ref><ref>{{Cite web |last=Richard |first=David |date=June 4, 2021 |title=Complete OBD2 Codes List With Basic Explanation |url=https://www.weeklytools.com/complete-obd2-codes-list-with-basic-explanation/ |website=Weekly Tools}}</ref> are five characters long, with the first letter indicating a category, and the remaining four being a [[hexadecimal]] number.<ref name="elm327dsl_pdf_p36">ELM327DSL.pdf, p. 36.</ref>

The first character, representing category can only be one of the following four letters, given here with their associated meanings. (This restriction in number is due to how only two [[bit|bits]] of memory are used to indicate the category when DTCs are stored and transmitted).<ref name="elm327dsl_pdf_p36" />
* P – Powertrain (engine, transmission and ignition)
* C – Chassis (includes ABS and brake fluid)
* B – Body (includes air conditioning and airbag)
* U – Network{{efn|Whilst this is commonly referred to as the network category, it may originally have been the 'undefined' category, hence the use of the letter 'U' rather than 'N'.}} (wiring bus)
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The second character is a number in the range of 0–3. (This restriction is again due to memory storage limitations).<ref name="elm327dsl_pdf_p36" />
* 0 – Indicates a generic (SAE defined) code
* 1 – Indicates a manufacturer-specific (OEM) code
* 2 – Category dependent:
** For the 'P' category this indicates a generic (SAE defined) code
** For other categories indicates a manufacturer-specific (OEM) code
* 3 – Category dependent:
** For the 'P' category this is indicates a code that has been 'jointly' defined
** For other categories this has been reserved for future use

The third character may denote a particular vehicle system that the fault relates to.<ref name="obdadvisor_codes" />
* 0 – Fuel and air metering and auxiliary emission controls
* 1 – Fuel and air metering
* 2 – Fuel and air metering (injector circuit)
* 3 – Ignition systems or misfires
* 4 – Auxiliary emission controls
* 5 – Vehicle speed control and idle control systems
* 6 – Computer and output circuit
* 7 – Transmission
* 8 – Transmission
* A-F – Hybrid Trouble Codes

Finally the fourth and fifth characters define the exact problem detected.