Retrieving Codes (With "Scan" Tool)

NOTE: To read trouble codes and check system voltages on serial data line, plug "Scan" tool into ALDL.

The "Scan" tool 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" tools may also furnish information on status of output devices (solenoids and relays). 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 checked 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 may not be indicated by all makes of "Scan" tools.

If trouble code is not present, this is not an indication that is no 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.

Out-of-specification sensors WILL NOT set trouble code, but WILL cause driveability problems. Using "Scan" tool is the easiest method of checking sensor specifications and other data parameters. Intermittent wiring problems may be identified by wiggling wiring harnesses and connections (key on, engine off) while observing "Scan" tool.

NOTE: If erroneous voltage signals are suspected, it will be necessary to verify tester information using 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". The same codes should be retrieved whether "Scan" tool or TRANS or SERVICE ENGINE SOON light is used.