CAR CLINIC | How to use vehicle diagnostic codes to your advantage, Part 2

17 May 2023 - 14:57
By Gerrit Burger
With onboard diagnostics, finding the cause of issues is made easier.
Image: parilovv / 123RF With onboard diagnostics, finding the cause of issues is made easier.

Your seven-year-old car had been exceptionally reliable. Recently out of warranty, it had never put a foot wrong and you were looking forward to many years of faithful service. But then something changed. It began with a brief stutter of the engine at odd moments, progressed to occasional hard starting (virtually unknown on modern cars), followed by incidents of the engine switching off randomly and refusing to restart immediately. 

At the same time, the usually silky smooth idling became rough and the engine lost some of its punch. And the “check engine” light on the instrument panel switched on.

This is the sort of situation where you are thankful for the onboard diagnostic (OBD) system fitted on modern cars. The symptoms described above can be caused by a multitude of problems, from faulty plug leads to loss of fuel pressure. Pinpointing the cause can be a daunting, time-consuming task requiring specialised testing equipment. What saves today's home mechanic from this ordeal is whenever a malfunction indicator lamp is turned on and/or the engine is put into limp mode, a digital trouble code is logged on the computer. Once this has been retrieved, it will provide valuable information that will allow you to narrow your search considerably. 

Various scan tools are available which can be plugged into the female OBD connector on a car to read the trouble codes They vary widely in sophistication and price. Probably the best compromise between affordability and capability is a so-called “dongle” which plugs into the OBD connector and communicates wirelessly with your cellphone. With the correct app installed on your cellphone, the trouble codes stored on the computer can be downloaded. I understand a functional dongle can be purchased for about R350 and it seems some people leave such a device permanently plugged into the OBD port. 

Let's return to the example of the car with the mysterious engine cut-out. Suppose the trouble code retrieved from the computer was P0335. To decipher this code, if the scan tool or cellphone app doesn't do it for you, you would have to consult a list of DTCs available on the internet. It turns out that code P0335 indicates a fault in the crankshaft position sensor circuit. This sensor supplies information to the engine computer about the exact position of the crankshaft at any given moment, as well as about the rotational speed (revs per minute) of the crankshaft. The computer uses this information to decide when exactly the spark plugs should fire or the diesel injectors should open. If the information is muddled or unintelligible, the spark timing will go awry, leading to misfiring or stalling. 

Notice that code P0335 points to a fault in a sensor circuit, not necessarily in the actual sensor. The circuit comprises, besides the sensor itself and the toothed timing rotor, the wiring and its connectors. Moreover, a worn timing belt will allow the camshaft timing to move out of sync with the crankshaft, leading to a discrepancy in the information received from the crankshaft position sensor and the camshaft position sensor. The computer, none the wiser, might flag both sensors. So the timing belt should also be checked. It's also possible that the fault lies inside the powertrain control module (PCM).  

For this part of the fault tracing process, the home mechanic has to depend on the internet and support groups such as car clubs and owners' forums as most manufacturers no longer supply the excellent workshop manuals of yesteryear.

Hopefully, Google will take you to a video or diagram showing the position of the crankshaft position sensor on your car's engine. This will enable you to inspect the wiring and its connectors. To check the signals emanating from the sensor you will have to call on an auto electrician. Only after every other possibility has been eliminated, will the suspicion fall on the PCM. 

This, then, is how the committed home mechanic has to roll in 2023. OBD is undoubtedly a valuable help, but in most cases only up to a point. 

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