Check engine light diagnostics

Feb. 12, 2014

Craig Truglia is an ASE A6 and A8 certified technician who presently works as a service writer for Patterson Autobody, a repair facility in Patterson, N.Y. A former shop owner and editor of several automotive repair magazines, Truglia combines his Columbia University education with the real-world experience he sees daily in the automotive repair field. Technicians Truglia and Fred Byron took part in diagnosing the different vehicles in this article.

For most auto repair shops, diagnostics are not where the money is. A good diagnosis is needed to figure out what parts (and these days, sometimes software files) are needed to repair a drivability issue.

It is safe to say that the most profitable field of diagnostics involves the skills and knowledge necessary to figure out what is required to allow the monitors to pass a state emissions inspection. As states such as New York adopt CARB emissions standards, emissions diagnostics may become increasingly important (yes, that means no more 49-state legal catalytic converters as of 2014).

Many experienced technicians get needlessly flustered diagnosing check engine lights. With the right diagnostic strategy that takes advantage of service information and proper procedure, many routine mistakes and time wasters can be avoided.

Step 1. Scan codes using generic OBD II.

Don’t waste timing booting up a big and expensive scan tool, unless it is the only one in the shop (or all the repair information resources are already on the tool.) Let’s be honest — sometimes we can almost complete a diagnostic by seeing a DTC alone (see Figure 1). Misfire codes on Fords, due to bad ignition coils, come to mind.

Many technicians make the mistake of using OEM enhanced functions on a scan tool as opposed to generic OBD II to diagnose check engine lights. There are two problems with this. First, it takes too long. Second, some aftermarket scan tools have a tendency to say there are current DTCs that really are not current. No one wants to chase down a code that is not going to turn off the light.


The reason many scan tools do this is that many OEM scan tools (such as GM and VW) have very sensitive criteria for picking up pending codes.

Now, this can be very useful in some circumstances, but also very confusing in others, especially if the “additional” DTCs are from a problem or event that is months old and not relevant.

Furthermore, when the technician actually begins diagnosing the vehicle using PIDs (parameter identifications) on his scan tool, OEM enhanced tends to substitute values for sensors in certain situations.

For example, a bad ECT sensor might read -40 degrees Fahrenheit in generic OBD II, but in OEM enhanced this may read as a normal temperature. For reasons such as these, do not use OEM enhanced functions until all the options generic OBD II offers are exhausted.

Don’t forget to check Pending DTCs when the scan tool is plugged in. Huge amounts of time can be saved with this information. For example, a misfire DTC coupled with a pending P0420 DTC would indicate either a misfire created by an exhaust back-pressure issue, or more likely, a misfire that has ruined the catalytic converter. It always helps to have an advanced heads up on things like this.

Furthermore, it cannot be emphasized enough how important Freeze Frame information can be. On many 1.8L Toyotas with a P0171, Freeze Frame is often the clue that allows the technician to suspect if there is a bad MAF sensor or a leaking intake manifold gasket. Oftentimes, vehicles that set the DTC when cold involve a very hard-to-catch leaking intake manifold gasket.

When diagnosing P0420 DTCs, the diagnostic can be done in a matter of minutes simply with the help of Freeze Frame. If a P0420 exists and long-term fuel trims are elevated positive or negative, the first thing that has to be diagnosed is why the engine is running lean or rich and thereby killing the converter (for an example, see Figure 2).

Now, more commonly, if long-term fuel trim is not above +/-7, it is time to check two different things: rear oxygen sensor operation and technical service bulletins (TSBs) for PCM reflashes. Don’t get dazed looking at anything else, even the front O2 or air-fuel ratio sensor if long-term fuel trims are good.

Long-term fuel trims would be off if these sensors, or the things that affect them such as vacuum leaks, were causing issues (see Figure 3).


Step 2. Make use of automotive repair information resources.

Information resources are indispensable when it comes to diagnosing vehicles. The most important thing an information system offers technicians are technical service bulletins (TSBs). Oftentimes, the OEM admits that there is a very common problem on a vehicle, or the TSB tells you what to test in order to figure out what is wrong in a list of several different common problems. Mitchell ProDemand (see Figure 4) is one such popular information resource. It allows technicians to look up TSBs, features repair tips within OEM repair information, and has wiring diagrams that are in many respects easier to use and manipulate than the OEM ones.

Illustrative of this is an issue on a 2008 Ford F250 6.4L diesel, as seen in Figure 5. The vehicle has a P2084, which is a DTC for a “side 2” exhaust gas temperature sensor. These sensors are used to calculate the efficiency of the DPFs (diesel particulate filters.) On these Fords, all these sensors have the same part number and are located one after another, as seen in Figure 6. These 6.4L diesels tend to have DTCs related to them at very low mileages. On this pickup, it had a P2084 at less than 20,000 miles.

The TSB informs us simply to “replace the faulty EGT sensor based on the DTC present.” That’s right. No test plans or anything. Apparently these EGTs drop like flies, but without working on tons of diesels, this might have not been obvious to many technicians.

Obviously it is not good to simply succumb to an OEM parts-changing routine, as many astute technicians would know that far too many modules have been needlessly replaced that way. So, after looking up service information such as TSBs and test plans, it is wise to start performing tests on the vehicle.


Step 3. Perform tests, beginning with PIDs on a generic OBD II scan tool and working one’s way up to other tools, such as a labscope, compression gauge, etc.

Now it is time to start testing the vehicle. While the first two steps are elementary and by the book, the third step is at the discretion of the technician (and the tools he has at his disposal). Diagnosing a check engine light at this point can be simple, such as swapping a coil and checking mode 6 to see if the misfire counter is changing to the cylinder in which the ignition coil was moved to. It can also pose difficulty, such as the need to trace wiring back to a PCM, or checking sensor waveforms. No matter what the test is, the idea is always to definitively prove something is good or bad, so as to narrow down the list of possible culprits.

Narrowing down the list is the most important thing about diagnosing a vehicle. This is why it’s wise to start with looking at Freeze Frame and TSBs before beginning step three. Otherwise, the list of possible culprits is so large, it is simply not time efficient to test everything.


Now, returning to the example of the 2008 Ford F250 6.4L, the technicians simply looked up repair information and found that Ford recommends looking at all the sensors at room temperature and condemning the one that does not match the temperature of the other ones.

It is important to note that the technicians working on this vehicle do not work on a lot of diesels, nor have they experienced this problem before. Yet, by sticking to the basics of diagnosing a check engine light, they were able to nail the problem.

A quick look at PIDs showed that all the sensors had identical temperatures. Being that the sensors are all identical part numbers, a meter was used to check ohms on each one.

However, being that none of the sensors were failing, the results were all identical. The simplest way to diagnose the issue was to swap the sensors and see if the DTC would change positions, but from a time perspective this was not a good idea.

Instead of doing that, the sensors were simply graphed with a scan tool for about 15 minutes until one of them started acting up for a very short period of time.

Not surprisingly, the one that the TSB said would be bad had a temperature of almost 1,000 degrees F and a voltage of almost 5V. See Figures 7 and 8 for an example of this. As seen on a Launch X431 scan tool, the Bank 1 Side 2 EGT sensor here is caught in the act of giving the PCM inaccurate data, obviously causing the P2084 DTC.

Now, this is when common sense needs to kick in. The TSB says that the sensors go bad all the time and the chances of a short to power somewhere further up the circuit was exceedingly low, especially because the vehicle was so new and a true short to power would have resulted in a reading closer to 5V.

The reason why it would be silly to assume that the PCM is faulty is because bad PCMs usually have bad drivers or consistently inaccurate PIDs with accurate sensor feedback. Being that none of these were the case, the sensor can be installed with a very high degree of confidence that it will fix the problem.

Next time you have a check engine light, try applying these three simple steps to your diagnostics.

A lot of time can be saved and maybe a headache avoided.   ●