DIY Network

Check-Engine Light and Scanner Diagnostics From "Weekend Mechanic" episode WKM-204

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The "check engine" indicator is a driver's safety net. It serves to alert the driver regarding any of hundreds of potential electronic or mechanical problems with the vehicle. The next step -- determining what the problem is -- requires some special diagnostic tools. Figure A Figure B Figure C Figure D Figure E Figure F Figure G Figure H Figure J

One of the most common reasons for bringing vehicles to auto-repair shops is the "check-engine" light being on. There's a good reason for that. The check-engine light alerts the driver of a myriad of potential problems -- some of them potentially serious -- based on the vehicle's onboard diagnostic system. The question is what to do next.

In this episode, Weekend Mechanic Bruce Bonebrake diagnoses the possible causes of a check-engine light remaining lit on the car's instrument panel. To do this, he uses a scan tool and electronic (computer-based) diagnostic aids. The scan tool indicates that the problem is with two of the car's electronic sensors. Further testing is done to determine whether the fault lies with the sensors themselves or with related electrical or mechanical components.

The problem: On our project car, a "check engine" light has illuminated on the car's dash, and is remaining lit. This is a warning indicator, and there are a number of possible causes for the light to come on. In modern vehicles, a check-engine indicator calls for the use of computerized diagnostic systems to help pinpoint the problem.

DIY Difficulty Rating for Repair: Since the focus here is on diagnostics, it's hard to assign a difficulty rating until the problem has been identified. We gave this procedure a difficulty rating of 2 on a 5-point intermediate-level scale. Depending on the nature of the problem, this job might be finished in a single day. Allow yourself two days to allow for any possible complications.

Important: Included below are the steps in this automotive procedure as shown in the episode. These steps are general guidelines that are applicable to most vehicles. With any particular vehicle, there may be procedures, specifications, settings, tolerances, components, etc. that are specific to that vehicle. There are also variations according to the type and brand of repair-kit you select. Always consult your vehicle's service manual when undertaking significant automotive repairs, and read and follow the manufacturer's directions and precautions that come with your kit or replacement parts.

Safety Alert: Always wear eye protection, and follow proper safety precautions, when working with power tools.

Safety Alert: As a standard safety precaution, always disconnect the vehicle's negative battery cable before you begin work on any electrical or mechanical components.

In addition to standard auto-mechanic's tools, some of the specialty tools that will be needed for this project include a volt/ohm meter, soldering gun, rosin-core solder, butt-connector, heat-shrink tubing, and a heat gun .

Diagnostic Tools

• Since the early 1980s, cars have been equipped with advanced electronics systems including onboard computers that help diagnose and alert for various mechanical and electrical problems.

  
  

• When a "check-engine" light comes on, the vehicles diagnostic system has detected a problem. Not only does the system provide an alert, but it retains in memory the nature of the problem in the form of a diagnostic trouble-code. To access that coded information, various types of diagnostic devices are used. One of the most commonly used device of this type is a dedicated scan tool. Scan tools (figure A) plug into a connector -- typically under the dash or somewhere under the hood -- to access and read the digital trouble codes.

  
  

• Software, cables and adapters for hand-held PDA's (personal digital assistants) are also available. A properly equipped PDA (figure B) can perform essentially the same function as a dedicated scan tool. PDA scan programs typically cost around $300.

  
  

• Similarly, software and adapters are available for laptops and personal computers (figure C). This is the type of system used in our demonstration. If you work on late-model cars, a computer-based scan system is a worthwhile investment.

  
  

• Connecting the laptop to on our vehicle was easy. A cable connects from the serial port of the laptop to an adapter (figure D).

  
  

• The adapter then plugs into a factory-installed connector beneath the dash on the driver-side (figure E).

  
  

• Once the connector is plugged in, an indicator light on the adapter (figure F) indicates that a good connection has been made.

  
  

• The cabling for the adapter is long enough that the laptop can then be taken outside the vehicle where the work is being done under the hood (figure G).

  
  

• With the ignition switch on the vehicle turned on with the key, the "check engine" light comes on. The trouble-code information pertaining to any potential problems should be stored in memory.

  
  

• With the laptop connected, and the scan program running, the computer loads all of the parameters relating to the car's system and potential problems.

  Note: There may be several thousand parameters, so it may take a while for the program to load completely.

  
  

• Once the program was operational, it displayed two separate trouble-codes that were stored into the vehicle's electronic memory (figure H). The first was for engine-coolant temperature, high input and the second for throttle position sensor, low input. Thus, there is an indication for potential problems associated with two sensors.

  
  

• The throttle-position sensor (figure I) is located at the rear of the engine compartment.

  
  

• The engine-coolant sensor (figure J) is mounted on the top of the engine, near the front of the engine and radiator.

In the segment that follows, work will proceed to determine whether the problems lie with the sensors themselves failing, or with associated mechanical or systemic problems. The throttle-position sensor will be the first to be tested.

RESOURCES :

The Complete Idiot's Guide to Trouble-Free Car Care
Author: Dan Ramsey
ISBN: 0028635833
Alpha Books

How Cars Work
Author: Tom Newton
ISBN: 0966862309
Publisher: Black Apple Press

Auto Repair for Dummies
Author: Deanna Sclar
ISBN: 0764550896