Automotive Diagnostic Tools

Little Problems, Big Failures

It’s almost always a little problem, but little problems can cause big failures. This is something I used to preach regularly to technicians I was coaching or training. Like most of you, I’ve seen my share of catastrophic failures, but when the failure is traced back to the origin, it is almost always found to be a relatively tiny flaw that snowballed into a larger problem.

Let me give you an example… In this case, we had a Crown Victoria that had a small under hood fire that appeared to start from the (DPFE) Differential Pressure Feedback Exhaust Sensor. 

This was a low mileage car that was less than a year old. I was sure this was something the factory reps would probably want to look into. I placed a call to our local Field Engineer and told the technician working on it not to repair the car until they had inspected it. They inspected the car, condemned the DPFE Sensor as the culprit, and repairs were authorized to replace the sensor and any fire damaged components. Parts were ordered, but I had that nagging feeling there was something else going on here.

Later that day, the technician approached me saying the Crown Vic will be ready to go after he clears a check engine light he found. That did not immediately seem unusual as I would imagine, with the sensor melted, there would likely be a Diagnostic Trouble Code registered. But I had a suspicion and asked him to let me know what he found. There were indeed codes related to the DPFE Sensor, but he also reported that he found a stored codeP0306, cylinder # 6 misfire detected. After clearing the codes the P0306 did not immediately return and our first thought was that the two were unrelated. In the service bay the car appeared to run fine. 

We were about to return the car to the fleet, but I couldn’t stop thinking about that misfire code and how it could possibly fit into the engine fire. Then suddenly, it hit me! I asked the tech to put the car on the hoist and drop the exhaust. “I want to see the cats.” I told him as he glared at me with that look of “Why?”. As I suspected, after the exhaust was lowered, inspection of the catalytic showed the substrate melted and almost completely blocking exhaust flow.

This made perfect sense now! A misfiring cylinder #6 overheated the catalytic damaging the substrate, the increase in exhaust backpressure forced more exhaust pressure into the DPFE eventually melting the sensor and hoses, and then continuing to spew hot exhaust under the hood, causing the small fire. Further, when tracking down the misfire, the only fault found was that the plug wire was not fully engaged onto the spark plug. This also made sense. The weakness in firing the #6 cylinder did not show up until under a load.

So a little problem created a big failure. A simple loose spark plug wire connection, led to an engine fire that could have easily destroyed the entire car. This car was nearly returned to service after replacing only the damaged parts, which only would have led to a second DPFE failure, or possibly worse! Taking the time to think about why the sensor melted in the first place, led us to more part failures, and eventually the “root cause”, the loose plug wire that started it all.

Many times we get busy and rushed, but take the time to go one step further and think about what caused the part to fail. The pay-off is well worth it! It may prevent the replacement part from suffering a similar fate; avoiding an unnecessary comeback, and a potential unhappy customer.

When diagnosing a car’s electrical system, this type of troubleshooting can be especially valuable, i.e. don’t just replace that melted relay, find out why it overheated! Is the load failing and causing too much draw? Are there any voltage losses in the circuit that would increase the current through the circuit? What is the small problem causing the large failure? Ideally, the entire circuit should be tested for any other electrical issues before simply replacing parts. Power Probe’s circuit testers are the perfect tools for this type of testing. You can check for voltage drops, load test components, verify powers and grounds, and with the smart, easy to use Power Probe testers you can quickly and accurately find those “little problems” before they can cause “big failures.”

I’m sure you guys and gals out there under the hoods have your own similar stories, even an instance where a Power Probe tester helped in your diagnosis. We can all learn something from them, so please let us know!

By David Barden