Scan Tester Usage

NOTE: Before connecting "Scan" tester, check diagnostic system and ensure accurate information is received by "Scan" tester. Perform appropriate DIAGNOSTIC CIRCUIT CHECK in BASIC TESTING article. If vehicle does not pass diagnostic circuit check, information received by "Scan" tester may be invalid.

The "Scan" tester is a specialized tester can diagnose on-board computer control systems by providing almost instant access to circuit voltage information without crawling under dash or hood to back-probe sensors and connectors. "Scan" testers reduce diagnostic time by furnishing input data (voltage signals) which can be compared to specification parameters. See SCAN DATA table in this article.

"Scan" testers also furnish information on output device (solenoids and motors) status. However, status parameters are only an indication output signals have been sent to devices by the ECM. They do not indicate whether devices respond properly to that signal. This must be verified at output device using a voltmeter or test light.

NOTE: Code 12 should always exist when ALDL is grounded with key on and engine off, but it may not be indicated by all makes of "Scan" testers.

If trouble codes are not present, a problem may still exist. Driveability-related problems with codes displayed occur about 20 percent of the time, while driveability problems without codes occur about 80 percent of the time. Out-of-calibration sensors WILL NOT set a trouble code, but WILL cause driveability problems. A "Scan" tester is the easiest method of checking sensor specifications and other data parameters. Tester is also useful in finding intermittent wiring problems by wiggling wiring harnesses and connections (key on, engine off) while observing data parameters. See SCAN DATA tables in this article.

NOTE: Information obtained by "Scan" tester is only as accurate as the tester itself. If erroneous voltage signals are suspected, verify tester information using a digital voltmeter and wiring schematic. If non-existent codes are displayed, turn ignition off and remove tester. Turn ignition on and ground ALDL test terminal. If same codes do not flashed by SERVICE ENGINE SOON light as were indicated by "Scan" tester, tester cannot be used on vehicle and information obtained by it will not be guaranteed accurate.