Engine performance issues were once diagnosed with the use of an oscilloscope and various meters and strobe lights. A skilled interpretation of the oscilloscope pattern was needed to determine exact ignition or compression conditions per cylinder. The advent of computer control devices ushered in the check engine light to alert drivers of emission control system failures. Advances in electronic engine controls allow in-depth testing of engine performance through the components of ignition and emission control systems.
On-board diagnostics on early computer systems required little more than a jumper wire to access the computer memory for "trouble" codes. The data exhibited was related to emission control sensors and devices, but did nothing to isolate or determine ignition system failures. Auto manufacturers all used differing versions, and the codes displayed were particular to the car make. These on-board diagnostic systems were called OBD-I, and they inspired federal legislation that obligated uniform codes.
Solid-state ignition components and fuel management devices became computer operated and monitored by OBD-II systems. The illumination of the check engine light can signify a problem with ignition or fuel delivery operations as well as emission controls. OBD-II systems can display trouble codes to basic code-reading scanners through the interface connector located under the dashboard. Basic code scanners are inexpensive, and some auto parts stores and service centers will retrieve your codes at no charge.
An inexpensive code-reading scanner will retrieve codes and shut off the check engine light, but the actual condition that created the code may require more intense scrutiny. Replacing a part that is named in a code interpretation is no guarantee of an effective repair. Sophisticated scanners display streaming data that aids precise diagnosis. High-quality streaming data scanners are expensive, and they are useless without extensive knowledge of automotive electronics.
An illuminated check engine light may signify a malfunction of a circuit or component by the code displayed. Related intermittent problems may cause the code to store in the computer memory, while the actual culprit eludes detection. An open circuit caused by a broken wire will set off the same code as a faulty component. Scanner operation instructions may provide some insight to proper usage, but true diagnosis depends on precise application of the data retrieved.