Many technicians have traveled down the wrong diagnostic path by reading OBD II diagnostic trouble codes (DTCs) with their scan tool and then replacing parts hoping to fix a problem. While this may work some of the time, it’s not the best way to address all engine performance problems. It’s important to remember that underneath all that technology, the majority of automobiles and light trucks are still powered by four-stroke, gasoline engines. And while there are challengers to the throne, for now, and in the near future, conventional gas-powered engines are still the majority of what comes into a typical shop or dealership, and they are still subject to many of the same mechanical problems that have existed for years.
Depending on the year, make and model, a vehicle’s Engine Control Module (ECM) can’t always set trouble codes for mechanical engine problems. Although OBD II diagnostic monitors relate in some way (and perform in response) to what’s coming out of an engine’s exhaust, or in the case of most ignition misfire detection, how evenly the crankshaft is rotating, they still are limited in their capability of diagnosing engine valves that don’t seat, a leaking head gasket or leaky intake manifold or exhaust gaskets. Surprisingly, if basic mechanical engine operation is out of whack a vehicle’s ECM still may not be able to respond to the problem and set the appropriate DTC, or any DTC.
One example of this is the
#P0300 DTC — Random/multiple cylinder misfire. Because the ECM (on most vehicles) senses the intervals between the engine’s power strokes, anything that effects rotational crankshaft speed could set the code. The most common causes are low fuel pressure, vacuum leak(s) or mechanical engine problems that cause low cylinder compression. Other possible causes are a faulty ignition coil(s), bad spark plugs or ignition wires, malfunctioning cam or crankshaft sensor, ignition module or the on-board computer. It’s ironic that the most common causes for a PO300 are not ECM-related, while the least common causes are all part of the engine management system, including the ECM and its array of sensors.
Engine mechanical problems have caused many an inexperienced — and some experienced — technicians to spend hours using a scan tool trying to figure out the reason why numerous DTCs have been set. Too often this diagnostic path leads to unnecessarily replacing components, only to eventually discover that an easy-to-fix vacuum leak, tired fuel pump, or not-so-easy-to repair bent valve(s) or leaking head gasket was the root cause of drivability problems and subsequent DTCs. Consequently, basic mechanical engine issues need to be isolated from computer-controlled engine management system components and their resulting DTCs before diagnostic hours are spent and/or parts are replaced.
In instances when an engine cranks over, but won’t start, there are some basic things that a technician can check. The first thing is obvious – use a scan tool to check if any DTCs have been set by the ECM. DTCs are a good place to start any investigation into potential causes of a specific problem, since they may provide valuable clues as to what the problem is, or at a minimum, in what system it is most likely occurring. For instance, if any of the range of
#P0350 to #P0362 DTCs are set, there is most likely a problem with the vehicle’s ignition system that is causing a no-spark condition. However, #P02XX fuel and air metering codes are also a possibility, so these should be checked as well. Moreover, remember, just because a code has been set by an ECM doesn’t mean it’s time to start replacing parts, at least not until it has been verified by other independent methods that a no-spark, or no-fuel, condition really does exist. Keep in mind that DTCs only provide a clue to the source of the problem, but not necessarily the answer.