Interstate Service Center - Mt. Joy
Interstate Service Center - Mt. Joy

Check Engine Light Diagnostics

     Your vehicle is driving erratically, or you’ve got a warning light on your dashboard that was blinking yesterday and now is on steady. What do you do? Hint: the answer is not to put a piece of electrical tape over the warning light and pretend it’s not happening. Quick attention to dashboard lights can save you thousands on serious repairs down the road.


What makes a dashboard light come on?


     Today’s vehicles are manufactured with complex interrelated systems. On-board computers continuously monitor and store information concerning the performance of these systems. A warning light comes on because the computer has sensed a signal from one of the systems or components that has gone outside of its normal operating range. The computer then sends a signal to turn on the dashboard light to let you know something is wrong!

     The potential causes for a warning light can range from improperly tightening your gas cap to engine, transmission or braking system performance issues. While dashboard lights can go on and the car still appears to be driving “fine”, ignoring some lights can lead to serious issues, such as engine damage or other component failure. In some instances, this can be a safety hazard. That’s why it is recommended to have your car looked at by a certified professional as soon as possible for both your safety and peace of mind. 


How can Interstate Service Center "diagnose" my problem?


     At Interstate Service Center, we have the tools, training and technology to properly handle diagnostic issues for everything from a Check Engine light to an ABS Warning light. Properly performing a complete diagnostic procedure normally includes the following steps:

1. RETRIEVE DATA – A full-functioning scan tool is used to perform an onboard computer system analysis in order to retrieve the following:

  • Computer codes
  • Freeze frame data
  • Computer software version numbers
  • Critical sensor data and status information

NOTE: Merely “pulling codes” does not generally provide enough information to make an accurate diagnosis of the problem. Pulling codes is the initial step in the process by identifying the circuit or system that has illuminated the vehicle’s dashboard light.

2. VERIFICATION TESTING – Verification tests determine if systems meet proper specifications. These tests may include an analysis of:

  • Engine vacuum and backpressure
  • Mechanical and electrical engine timing
  • Ignition system
  • Fuel system
  • Emissions system

NOTE: Failure in any of these basic systems can cause the check engine light to illuminate and falsely identify a circuit or component as the problem, which could lead to improper or unnecessary parts replacement if your garage does not complete verification testing.

3. PINPOINT TESTING – All manufacturers require that circuit or component pinpoint testing is done in order to make an absolute diagnostic determination. These actions involve:

  • Working through wiring diagrams
  • Performing electrical and/or mechanical tests
  • Performing bi-directional control tests
  • Visual inspections of suspect areas


4. RESEARCH TECHNICAL SERVICE BULLETINS – Technical Service Bulletins and hotline information will indicate if the problem has been determined to be a pattern failure, meaning that it is a known issue that occurs with a particular type of vehicle. This does not eliminate the need for testing, as other systems must meet specifications before a service bulletin procedure should be performed.


5. CAR REPAIR – This may involve multiple actions such as:

  • Parts replacement
  • Cleaning of passages, connections and/or components
  • Performing manufacturer technical service bulletin updates
  • Updating of vehicle’s computer software
  • Wiring repair or replacement


6. VERIFY REPAIR – Once all necessary actions have been taken, the vehicle must be retested to verify that the repair is complete and has achieved the proper results.

     - How long will it take to diagnose my problem?In most cases, we can actually find AND fix the problem in the same day.

     - Do I need an appointment?

Most centers do not require an appointment, but please call for details. It is our goal to make it convenient and offer times that works best for you!

     - How much will it cost?

Diagnostics are usually handled at an hourly or procedural rate. A Check Engine Light diagnostic will often be at a fixed rate, unless additional diagnostic procedures are needed.


Why does my parts store offer to "do my check engine light" for free?

     Parts stores capabilities are limited to only reading the stored trouble codes, but they do not perform the diagnostic procedures to determine what is exactly required to fix your vehicle. Interstate Service Center has certified technicians whose job it is to accurately diagnose and fix cars!

Do you have any questions about our services? Please contact us - We look forward to hearing from you!

4 Convenient Locations!

Mt. Joy, PA

Interstate Service Center

1125 Strickler Road.

Mt. Joy, PA 17552

Phone: 717.287.1120

Fax: 717.287.1121

Hours: 8-6 M-F 


Use our contact form.

Colmar, PA

Interstate Service Center

696 Bethlehem Pike

Colmar, PA 18915

Phone: 215.822.8115

Fax: 215.822.8587

Hours: 8-6 M-F 


Use our contact form.

Allentown, PA

Interstate Service Center

619 Union Blvd.

Allentown, PA

Phone: 610.289.6170

Fax: 610.289.6174

Hours: 8-6 M-F  


Use our contact form.

North Hills, PA

Interstate Service Center

3051 Mt. Carmel Ave.

Glennside, PA 19038

Phone: 215.576.5110

Fax: 215.885.3064

Hours: 8-6 M-F


Use our contact form.

Inspection Special

We perform state inspections. Call us to schedule an appointment today.

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