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Old 15-04-2017, 06:06 PM
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Holly Goodhead Holly Goodhead is offline
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Default Scan Tool Help

Scan Tool Help

A scan tool is a must for automotive diagnostic work today. When your Check Engine Light is on, you have to access the vehicle's onboard diagnostics with a code reader, scan tool or scanner software to find out what's wrong. A scan tool allows you to read faul codes and other diagnostic information.


On most 1995 and older pre-OBD2 domestic vehicles, diagnostic trouble codes can be read manually by grounding or jumping certain terminals on the vehicle's diagnostic connector. This puts the powertrain control module (PCM) into a self-diagnostic display mode, causing it flash out the code via the Check Engine Light (Malfunction Indicator Lamp or MIL). You then look up the code number in a reference chart (such as DTC Code Finder) to find out why the light is on.
The problem with reading manual flash codes is that (1) they are no longer used on most 1996 and newer vehicles (one exception is Nissan), and (2) counting the series of flashes can be confusing. Most flash codes use a combination of long and short flashes to indicate double digit codes, and if the vehicle has more than one code, it may be tricky to tell when one code ends and the next one begins. So the preferred method of reading codes on older vehicles is to use a code reader or scan tool.

On some 1996 but 2000-2001 and newer vehicles with OBD2, there are no manual flash codes. You must have a code reader or scan tool to read the codes.


The most basic diagnostic tool is a code reader. A code reader can access and display codes from your vehicle's computer. The least expensive models only display a number while the better ones also provide a definition (some are even bilingual and can display in English, Spanish or French). Code readers typically sell for under £50.

A code reader can also clear codes to turn off the Check Engine light. Some code readers can also display the "ready" status of various OBD II monitors (ready means the monitor has completed its self-check process). But a code reader is NOT a scan tool because it only reads and clears codes. It does NOT display any sensor data or other system operating information. To read sensor and other system data, you need some type of scan tool or scanner software.
An important point to keep in mind here is that a fault code by itself does NOT tell you which part needs to be replaced. The code only tells you that a fault has been detected, not what caused it. The code serves as a starting point for further diagnosis. Many people don't understand this and assume an inexpensive code reader is all they need to "diagnose" and repair their vehicle.

Also, don't assume all code readers display all codes. They all display "generic" or "global" OBD2 codes ("P0" codes). But some do not display manufacturer "enhanced codes" ("P1" codes), or if they do, the list of codes may be limited to domestic vehicles and not include any enhanced codes for Asian or European vehicles.

Something else to check before you buy is the model years the code reader can access. Most code readers are for 2000-2001 and newer OBD2 vehicles with a standard OBD2 16-pin connector. deisels 2005-on. Most code readers cannot read codes on 1995 and older cars or trucks because the connectors are different. However, vehicle-specific code readers are available for older applications. The same is true for BMW, MINI and some other import applications.

Something else to keep in mind about code readers (and scan tools), is that the list of new DTCs and system data grows with every new model year. Last year's tool may not work on next year's models. Tools get out-of-date VERY quickly, and have to be updated with new software via plug-in memory chips, cartridges or internet downloads from the tool supplier. If you are shopping on ebay for a used code reader or scan tool, make sure it will work on your vehicle, or can be updated to your vehicle.


For advanced diagnostics on today's vehicles, a full feature scan tool is an absolute must. Scan tools for do-it-yourselfers can display sensor values and system data, but DIY scan tools cannot perform various system self-tests such as checking the operation of the fuel pump, cooling fan(s), idle speed control motor or solenoid, EGR solenoid, A/C compressor clutch, fuel injectors, EVAP leak test, EVAP purge controls, etc. This level of diagnostics requires a professional level scan tool (which are EXPENSIVE!) with bidirectional (two-way) communication capability and the proper software for accessing and running these type of tests.

Scan tools have different ranges and capabilities. Entry level "generic" scan tools typically sell for less than £100. They can read and clear codes, display the status of the various OBD II system monitors, and display basic operating data such as loop status (Open or Closed), airflow, coolant temperature, oxygen sensor outputs, throttle position and other sensor readings, and fuel trim values for diagnostic purposes. Most of these tools are fairly versatile and work on all domestic makes (Ford, GM & Chrysler), but may require additional software for Asian and/or European applications.

Entry level scan tools that are sold in auto parts stores are usually designed for do-it-yourselfers, and lack bidirectional communications capability for liability reasons. They may also display only a limited number of "PIDs" (Performance Information Data such as sensor values, switch status and other operational data) compared to a professional level scan tool or factory scan tool used here.


This help topic is subject to changes without notification. The information within is carefully checked and considered to be correct. This information is an example of our investigations and findings and is not a definitive procedure. The Sixties Garage accepts no responsibility for inaccuracies. Each vehicle may be different and require unique test settings.

Last edited by Holly Goodhead; 18-04-2017 at 01:47 PM.
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