Diagnosing a Check Engine Light - Everything You Need to Know

Video tutorial on how to scan and erase your check engine light. A check engine light or also known as a cel for short is a fault with your vehicle’s engine which will illuminate a light on your gauge cluster as shown here. When a fault is present, this light has two modes. One will be solid or stay on all the time or blink. When the light is solid, there is obviously a problem but it’s not an immediate issue. That doesn’t mean it shouldn’t be fixed either as the fault can cause increased damage depending on what it is. For a blinking cel, this means there is an immediate issue where the vehicle should not be driven and the vehicle must be fixed immediately.

Tools/Supplies Needed:

• OBD2 scanner/code reader

To determine if the check engine light is working, you can turn the key onto the run position, there is no need to start the vehicle and this will illuminate the whole gauge cluster showing the functioning lights. If your light does not illuminate, it is either burnt out or someone has removed it. Someone may remove it to hide faults, a dishonest practice which is done to help sell a vehicle.

First we will locate the diagnostic port. This is an ODB2 vehicle, which means his is the second generation on-board diagnostic system. This is a standardized system, much different than compared to OBD1. This system has a generalized plug area, usually within 3’ feet or 91cm of the steering wheel, generic plug, and they share the same fault code meaning. This system of was a set standard in the automotive world in 1996, but you may find vehicles a year or two before with the system depending on the manufacturer.

For this vehicle which is a 1997 BMW 540i, the plug is located under the steering column on the left side.

Here is a 1998 Ford Ranger, this also shares a similar area, this time towards the center directly underneath the steering column covered by a cap.

Once you’ve located that port, plug in the diagnostic tool. The plug only connects in one orientation.

Sometimes the ignition can be left off, put in the accessories position, run position, or the vehicle will need to be running. I find this depends on the vehicle, the scanner will turn on, but won’t necessarily read the codes which will be promoted with a connection error. So start with the vehicle off first, then slowly work your way up to each ignition position until the codes can be scanned.

Read the vehicle, for this scanner we can press the arrows on the key pad to select the OBD2 scan, press enter and the computer will be scanned for any fault codes. For a diagnostic tool, here I am using one from AutoPhix, model OM126.

Once the scanning is complete, select the down arrow, and then read the codes. DTC stands for diagnostic trouble codes, we may either have current codes which was just scanned, pending codes which are faults stored in the computer that either haven’t occurred enough times or is on the path of being erased by the vehicle’s computer and permanent is a high priority error which will illuminate up the MIL or also known as the malfunction indicator lamp.

Select enter. For this specific code reader, the screen will prompt you to selecting what type of vehicle you have so the fault code definitions are specific to this vehicle.

We can see on the top right corner I have 4 fault codes. We are currently on 1 out of 4 codes. This being P0505 which is an Idle Control System error.

P1174, Fuel trim adaptation, additive bank 1 malfunction.

P1175 Fuel trim adaptation, additive bank 2 malfunction.

And P0420 Catalyst System Efficiency Below Threshold Bank 1.

Going thought the other fault areas on pending and permanent. When there is an error in the vehicle’s computer, this can affect anything from acceleration, overall performance, create hard starts, decreased fuel economy, or making an engine run rough. Other times depending on the vehicle’s parameters, it maybe something so small you don’t notice any change in how the vehicle runs, but it’s just enough to trigger an error.

Next we can exit out of the fault readings and clear the codes if desired. Using one of these tools to scan your vehicle is an assist to help with repairs and doesn’t necessarily tell you the exact fault. For instance, the first fault code P0505 cause. While this relates to the Idle Control System, this entails that the engine idle isn’t staying within factory specification. So this could be the idle control valve is sticky which needs to be cleaned, or perhaps it’s slowly failing. There could be a vacuum leak, that would be a cracked vacuum line, bad gasket, or a clamp needs to be tightened. Issue with the mass airflow sensor, and so on. When there is multiple codes such as the next two which can relate to the first one, one error maybe causing all these codes. Something that would relate to all of these faults would be a vacuum leak for example.

And even if there isn’t a check engine light, doesn’t mean there isn’t an error with the vehicle. The vehicle’s computer will store the code for a certain period of time, which is a pending code. If the fault disappears the code will eventually erase. If the fault persists, the will trigger the light.

If you don’t fix the persisting fault, that code will keep coming back. Once those codes are erased, as you can see there is no check engine light.

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