Intermittent Diagnosis - Check Engine Light Help

So you've got a check engine light concern that only happens sometimes? There are a few things we can do to help duplicate and find the concern. Typically an intermittent check engine light concern happens under very specific conditions such as when it's hot, cold, damp or driving over bumps. Knowing this information can greatly reduce the time it takes to locate the source of the fault. The first thing we need to do is duplicate the concern. So if the fault only happens when the vehicle is hot, run it up to temp or check it on a hot day. If it only happens after it rains then get out the hose and duplicate a rainy day, or drive it through a carwash!

For concerns that happen over bumps we can drive over a bumpy road and wiggle test harness and connectors while trying to duplicate the problem. Testing for an intermittent problem is useless if the actual symptom is not present . The hardest part of these check engine light issues is re-creating the fault, but it is also the most important! Once we have duplicated the problem and the vehicle is actually exhibiting the concern then we can begin testing the circuits and components related to the check engine light codes or driveability issues. See our article on Automotive Circuit Testing, for help with this. If you just can't seem to duplicated the symptoms here are a few things to try.

First, determine what vehicle system is likely causing the problem and focus on these components or circuits. For instance if you had a skipping concern at highway speeds, we would probably focus on the fuel and ignition systems first and then maybe follow up with the EGR system. We wouldn't likely spend time on other vehicle systems at first. Once we have an idea of what to check we would start by wiggle testing all connectors and harnesses in that system in an attempt to duplicate the symptom. We would then "shock test" (gently tap), suspected sensors/solenoids and if all else fails, we could remove the suspect components and try to duplicate the conditions they fail under. We would then component test them, reinstall and recheck system function.

For instance if we had a Mass Air Flow Sensor we thought was acting erractically on cold mornings, we could remove it, put it in the fridge to bring it down to temperature and see how it reacts when re-installed in the vehicle. You would be surprised how quickly you can find concerns in this manner. Persistance pays when trying to duplicate a concern. If you have a check engine light on then you will likely have codes in memory that can help you determine what system to start your diagnosis with. If there are no codes, then spend some time determining what systems are most likely to cause your driveability or other concern. Time spent determining probable fault locations can save you hours on wiggle testing for fault duplication. Some intermittents, like ones that happen once a month under no specific conditions can be very difficult to pinpoint, but with some patience and a little research, they can all be fixed!

So to recap, intermittents are usually caused by one or more of the following conditions: poor electrical connections, terminal tension or other wiring problems. Start by carefully inspecting the suspected circuit for the following conditions:

Poor mating of the connector halves
Terminals backed out or not fully seated in the connector
Damaged terminals or excessive connector tension
Poor terminal to wire connections
Corrision or water intrusion. Pierced or damaged insulation can allow moisture to enter the wiring causing the conductor to corrode inside the insulation. Look for swollen and stiff sections of wire in the suspect circuits.
Wires that are broken inside the insulation
Pinched, cut or chaffed wiring in the harness
Wiring that is damaged as a result of being in contact with exhaust components

Some electrical components or circuits are sensitive to Electromagnetic Interference (EMI) or other types of electrical noise. Inspect for misrouted harnesses that are too close to a high voltage or high current device such as secondary ignition components, motors and alternators. This condition may induce electrical noise on a circuit that could interfere with normal operation. Finally you should determine whether non-factory or aftermarket add-on accessories are installed in the vehicle that may be causing the intermittent. These may include lights, radios, remote starters, alarm systems or cell phones.

So, this wraps up our ideas on intermittent diagnosis for check engine light concerns. Please feel free to share your ideas or suggestions with us, and I hope you found this article helpful!

The Team

Back To Top Of Check Engine Light Intermittent Diagnosis Page