Automotive Circuit Testing 101 - Check Engine Light Help

Tackling the wiring portion of check engine light code diagnosis can be overwhelming without the right skills. I am always amazed at how many different ways automotive professionals rationalize and test automotive circuitry. Electrical concerns are actually my favorite repairs to make because you really get to think outside of the box.

And when dealing with check engine light concerns you get to do a lot of electrical testing! It's pretty cool to examine a schematic and narrow a concern down to very specific probable causes or locations. You can then go in to isolate and repair the problem in a very short (no pun intended) period of time. Today I would like to share my ideas on the most effective and efficient ways to approach circuit testing in today's cars.

Anyone up for some check engine light circuit testing? Let's start with the necessary tools required for most tests. These are very basic and consist of: a DVOM (Digital Volt Ohm Meter), test light and some test leads/jumper wires. The DVOM should have an ohm scale of at least 0-20 K Ohms, a voltage scale of at least 0-20 Volts and an Amperage Scale of at least 0-10 Amps. You will also need the required hand tools to gain access to the various connectors and components to be tested. There are a lot of tools designed to ease the testing process and speed things up, but these are not absolutely necessary. I recommend checking out some of my favorites through the "Tools" link if you do anything more than an occasional circuit test, as some of these testers can save you considerable time and frustration on those tough ones! Let's get this check engine light taken care of!

Automotive electrical testing as a whole is a very large topic, and often requires very specific test steps for things such as module and component testing. This being said about 90% of the testing required to diagnose check engine light concerns is very basic, and nearly all circuit concerns can be isolated using the same simple basic techniques. There are many different methods used for circuit testing. I am going to share the methods I have developed and use to pinpoint wiring issues in the least amount of time when specific testing instructions are not available (or needed for that matter). Keep in mind-you will run into some weird issues such as frequency interference along circuits, but these are beyond the scope of this article. We will cover all the basic testing needed to find 90% (or better), of all wiring issues in today's cars. So let's get started!

In a nutshell, wiring problems come down to only a few possible causes, and they are:
1. Open circuit (or partially open such as a chaffed line which causes high resistance due to heat when circuit load is on)
2. Short to power
3. Short to ground
4. High resistance (usually caused by corrosion or poor connections)

Testing for all cases is fairly straight forward once you have a schematic to deal with so you know what wires you need to test and where they run. We are going to use a simple lighting circuit to demonstrate how to test an individual wire for each of the above, and then we will troubleshoot a concern. I know it's not really a check engine light concern, but we're trying to get the basics down for now, and then we'll get into the good stuff! First take a look at the schematic:

Check Engine Light Circuit Testing Article Image

Got an idea how the system works? Let's cover a few basics on reading the diagram. The letter "C" refers to a connector, so C162 is connector number 162. "G" is a ground, so G104 is ground number 104. "S" is a splice, which is where multiple wires join a circuit, so S246 is splice number 246. Wires in the diagram that are a solid line are a solid color, and lines that have a dashed look are wires with a tracer color, so GN/Y is a solid green wire with a yellow tracer line running down it. You should be able to distinguish a fuse symbol from the image and switch symbols are fairly straight forward also. Switches are usually shown in their resting or normal position. The large dashed box around an item basically means "is part of" or "is located in" so the dashed box ground the 10A fuse indicates that it is located in the Instrument Panel (IP) fuse panel. One last lesson on relays: nearly all relays function the same. Take a look at the relay in this schematic. It has 5 pins: 87, 87A, 30, 85 and 86. This is very standard. You have power at pin 30 always and power on pin 85 when the headlamps are on low beam. The relay contact (resting normally on pin 87A) will be pulled to pin 87, powering the fog lamps when the coil is energized by grounding pin 86. This is done when the fog lamp switch is turned on and a path to ground is provided to G106. Nearly all relays work in this fashion, a coil is energized, closing the relay contacts and powering up the load side of the circuit. Being familiar with relay operation is very important in automotive circuit testing. Okay, so there are the basics. Let's get down to testing a circuit for the basic concerns we talked about, which will help us fix many check engine light issues!

We are going to test circuit 310, which as we can see runs between relay pin 87 at the connector 204 in the relay box and both right and left fog lamps. First we need to isolate the circuit, so we would disconnect connector 204 and connectors 162 and 130. First we'll test for an open circuit by setting our meter to the appropriate scale. Refer to your meter's owners manual, but this will likely be a 0-10 K Ohm scale. Next, hook your leads between the GN/Y wire for circuit 310 at connector 204 (verify correct wire if multiple wires of similar color enter the connector) and the other end of the wire at connector 162. We would expect to see less than 5 ohms (likely closer to .3 Ohms) on this line. If we have greater than 10 K Ohms or "OL" (Open Line), on our meter then we have an open circuit. Always double check that your meters connections are secure before condemning a circuit. At this point we can check for excessively high resistance as well. If our meter reads above the expected 5 Ohm reading but below an open line reading of better than 10 K Ohm, then we have excessively high resistance. This can be caused by corrosion in the circuit or connectors or possibly a short to another circuit in the harness. Check for these concerns and repair as needed. I have found that the quickest way to pinpoint the location of a wiring concern is to maintain one connection, lets say we leave the lead on C204, and move the remaining lead closer by testing up the harness. So if we had an open on circuit 310 between C162 and C204 then we would test between C204 and S246. If we still had an open we would know it was between S246 and C204 and that the wiring between S246 and C162 is likely okay. This greatly reduces the amount of time needed to isolate the problem area, and can be used when testing any wiring faults. Always try to use the easiest connectors to access when using this method as it simplifies the process and saves time. Okay, we need to test the other half of the circuit, so put your negative meter lead back at C204 and let's get back to testing.

We can now check the remainder of the circuit for an open by moving our test lead from C162 over to C130. This will tell us if the line between C204 and C130 is good. Again, we are looking for less than 5 ohms on this line. If you find a problem then check the wiring harness to locate the open wire and retest. Check engine light issues caused by open circuits are often caused by chaffed wiring harnesses or loose wire pins at the connectors causing intermittent or poor connection. So be sure to check for these. Check engine lights can also be caused by open circuits inside a sensor or even a computer or control module, so don't rule these out. If you did not find an open then check for excessive resistance as we did before and then we can go on to test for a short to ground or power!

Okay on to checking for shorts to power or grounds ( a common cause of check engine light concerns)! We'll be testing the same circuit, which has already been isolated as before by disconnection C204, C162 and C130. As a note, always be sure to disconnect any other connectors or components that may share a common splice point as we need the circuitry isolated completely for accurate results. We do want to get rid of that check engine light, right? So to check for a short to ground, we will want to put our meter's negative lead on the battery ground (using same meter scale as before) and connect our positive lead to circuit 310. You should be able to check at any point on the circuit for a reading, as we have already verified there isn't an open. Your meter should still read "OL" or open line. If you get a resistance reading (it would likely be around 5 ohms) then you have a short to ground, which means that your circuit is grounding out somewhere. This could mean that the wire is either chaffing against the body or engine, or perhaps another wire in the harness. Finding concerns like this keeps fixing the check engine light fun! To check for a short to power, leave everything hooked up the same but have your meter on the voltage scale (this will likely be a 0-20 Volt scale). If you get any voltage reading at all then you have a short to power. Find and repair as previously explained.

So that covers the basics of check engine light circuit testing and should provide you the skills needed to repair engine light related wiring issues (or any automotive wiring issue for that matter)! But where do we go from here? Well there are some additional things to consider. This article on check engine light circuit testing was becoming somewhat lengthy, so we've split up the information a bit. To follow up on this repair info, I recommend reading out our newest article on Voltage Drop Testing. This last bit of info should give you the skills you need to perform any check engine light circuit testing! Thanks for taking the time to read our article, and I certainly hope you have found something useful!

The Team

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