Scan Tester Usage

NOTE: Prior to connection of "Scan" tester to vehicle, diagnostic system should be checked to determine if system is operating properly and if information received by "Scan" tester will be accurate. This is done by performing DIAGNOSTIC CIRCUIT CHECK located in BASIC TESTING article in this section. If vehicle does not pass diagnostic circuit check, information received by "Scan" tester may be invalid.

The "Scan" tester is a specialized tester which, when plugged into ALDL, can be used to diagnose on-board computer control systems by providing instant access to circuit voltage information without need to crawl under dash or hood to backprobe sensors and connectors. "Scan" testers cut down diagnostic time dramatically by furnishing input data (voltage signals) which can be compared to specification parameters. See tables under SCAN DATA in this article. They may also furnish information on output device (solenoids and motors) status. However, status parameters are only an indication that output signals have been sent to devices by the control module. It does not indicate if devices have responded properly to that signal. This will need to 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 not running, but may not be indicated by all makes of "Scan" testers.

If trouble codes are not present, this is not an indication that there is not a problem. 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. Sensors that are out of specification WILL NOT set a trouble code but WILL cause driveability problems. Using 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 tables under SCAN DATA in this article.

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