Scan Tester Usage

NOTE: Before connecting scan tester to vehicle, diagnostic system should be checked to determine if system is operating properly and if information received 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 may be invalid.

Scan tester is a specialized tester which, when plugged into DLC, 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 tester cuts down diagnostic time dramatically by furnishing input data (voltage signals) which can be compared to specification parameters. See SCAN DATA. They may also furnish information on output device (solenoids and motors) status. However, status parameters only indicate output signals have been sent to devices by control module; they do not indicate whether devices have responded properly to signal. Verify proper response at output device using a voltmeter or test light.

Vehicles utilizing a 16-terminal DLC or 12-terminal DLC with terminal "B" not used, must use a scan tester for retrieving codes or enter Field Service Mode.

NOTE: Code 12 should always exist when DLC terminal "B" is grounded with key on and engine not running, but it may not be indicated by all makes of scan testers.

A problem may exist even if trouble codes are not present. About 80 percent of driveability problems occur without trouble codes. Sensors that are out of calibration 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 SCAN DATA .

NOTE: 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, remove tester, turn ignition on and ground DLC test terminal "B". Same codes flashed by SERVICE ENGINE SOON light should be indicated by scan tester.