Free check engine light basics: How to diagnose any fault codes and save money in auto/truck repairs (1985-2013)

check engine light codes and strategies in fixing it

Your check engine light is your first indicator if the vehicle has an engine problem however, you need to find first the meaning of that engine light code and it's equivalent repair.

When your check engine light comes on in your dash, this means that vehicle engine computer (ecm) has stored some electronic signal indicating that your engine has a problem that can affect engine performance. Example if the engine is misfiring, it will result in fault code P0300 when scanned; which when left unattended could damage your catalytic converter.

On earlier domestic vehicle models, if you get a code 32; it usually means your exhaust gas recirculation (egr) system is at fault and needs testing. On both occasions, the sequence of events was started when the check engine light came on. This is why as soon as you see it in your dash; you have to scan it to get the fault codes.

When the check engine light device was started, it was intended to help control vehicle emission to help protect our environment. A lot of technical electronic components were introduced and this system of having a light flashing in your dash is a quick way of warning the driver or mechanic if a malfunction occurs.

Started as an on board diagnostic (OBD1) system, it was used for vehicles made before 1995. Later it was upgraded to OBD2 for vehicles made after 1996; and made the diagnostic procedure more universal and uniform for all types of vehicles. To help resolve some confusion on how the check engine light is used, some important highlights are explained below.

Check Engine Light

It is the same as SERVICE ENGINE SOON light (SES) or Malfunction Indicator Lamp (MIL) indicator light which are displayed in your vehicle. When these lights come on, it does not automatically means that the engine itself is at fault.

If you accidentally touch a vacuum hose and got it disconnected, chances are it will set up an oxygen sensor code. A lot of times, the code could be triggered if you just changed the gas station to fill your tank which triggers a fuel tank mixture code. The proper way to analyze a check engine light is to scan for codes and analyze it base on the engine symptoms.

Diagnostic Connector

All check engine light problems could be retrieved by accessing the diagnostic connector where the scanner is to be hooked up. On older vehicle models, the diagnostic connector could be connected to a short ground wire or a simple test light.

On later vehicle models, the standard practice is to use a scanner and hook it up to your diagnostic connector. On earlier type such as OBD1 system, the diagnostic connector location will vary from one vehicle manufacturer to another. While on OBD2 system, the location is all located in the left side of the driver under the dash panel.

Free check engine light strategies:

As soon as you see the check engine light, you can reset it by following the vehicle manufacturer manual. A rule of the thumb is to disconnect the battery negative terminal for 1 minute and reconnect. This usually applies to the 2000 vehicles 5 or older but if the vehicle has newer model, you have to follow the service manual.

The resetting must be done to confirm if the initial code scanned is a HARD (permanent) code or SOFT (temporary) code. The hard code refers to problems that are more permanent and needs to be fixed immediately whereas the soft code as those problems which are temporary in nature. The latter can be fixed by resetting the fault code using the battery terminal disconnection or scanner.

Testing free check engine light codes:

When a hard code is confirmed, you have to associate the code to what the engine is actually doing like if you have a lean gas mixture oxygen sensor code for example:

Is the engine running rough?
Is the engine gas consumption bad?
Does the engine hesitate when accelerated?

A lot of times, the fault code itself is confirmed by the engine performance and if the example above is caused by a vacuum hose which is leaking, reconnecting it will solve the problem. Mind you we did not even change the oxygen sensor which will happen if you understand the underlying cause of the fault code.

The same approach can be used when testing the fault code and if you associate the meaning of the code first and then see if the symptom matches the problem. Once verified, then you need to do is perform the basic repairs associated with the check engine light problem.

Tips for check engine light repair:

  • For every check engine light code, there is usually a COMMON FIX which can be tested simply by wiggling the component or it's wiring harness, component integrity, connection, etc.
  • Getting the engine light code WILL NOT fix the problem unless you trace the circuit using a WIRING DIAGRAM to diagnose the problem.
  • A diagnostic scanner is only useful if you know what to look for and interpet the data shown.


All of the above will be given and shown to you one on one if you are a member of ATS

Please watch below one of technique ATS uses for diagnosing check engine light problem.

If you own a vehicle made before 1995, here below are free check engine light OBD1 codes used for different vehicle makes:

GM OBD1 codes

Ford OBD1 codes

Chrysler OBD1 codes

Nissan OBD1 codes

HONDA OBD1 codes

Toyota OBD1 codes

ACURA OBD1 codes

Hyundai OBD1 codes

LEXUS OBD1 codes

Mitsubishi OBD1 codes

ISUZU OBD1 codes

Suzuki OBD1 codes

Subaru OBD1 codes

Jaguar OBD1 codes

BMW OBD1 codes

Volkswagen OBD1

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