666 Auto Electricity and Electronics Copyright by Goodheart-Willcox Co., Inc. coil, spark plug, and injector, as well as for mechanical wear or damage, in that cylinder. Th e random cylinder misfi re code (P0300) indicates that a problem is occurring in all cylinders. You would know that something aff ecting all the cylinders is the cause. For example, an engine vacuum leak, restricted exhaust, or low fuel pressure may cause a random cylinder misfi re code. Remember that there are four conditions that can cause engine misfi re codes: • Low engine compression will keep the air-fuel mixture from igniting on the engine power stroke. Th e engine valves, piston rings, or cylinders could be worn or dam- aged. Th ese types of problems will usually aff ect only one or two cylinders. • Improper fuel delivery causing an extremely lean or rich fuel mixture can prevent normal combustion and a misfi re. A clogged or leaking fuel injector, improper fuel pressure, and similar problems could be aff ecting engine combustion. One bad injector will set a misfi re code for only one cylinder. Improper fuel pressure could aff ect the misfi re in any or all cylinders. A rich mixture will increase catalytic converter operating temperature. • A weak arc at the spark plugs can prevent normal com- bustion. If a spark plug gap is burned too wide from extended service, it can cause a misfi re in the aff ected cylinder. • A restriction in the engine intake tract or exhaust system can cause poor combustion. A dirty air fi lter, intake vacuum leak, or restricted catalytic converter or muffl er can cause an engine misfi re. Th ese problems can aff ect one or more cylinders. Keep these problems in mind when you analyze misfi re trouble codes. OBD II Misfi re Data An OBD II scan tool can produce the following mis- fi re data: • Misfi re data values—scan tool readouts that indicate something is causing an engine cylinder not to fi re its fuel mixture properly. Th e misfi re data can be recorded by the vehicle computer and stored in memory. Th e scan tool will retrieve this data and help you fi nd problem sources. • Misfi re history—indicates which cylinder was misfi r- ing and how badly it has been misfi ring. • Misfi re passes—shows how many times the cylinder has not misfi red. • Misfi re failures—indicates how many misfi re tests have been recorded. • Misfi re rev. status—shows accepted misfi res (real mis- fi res) and rejected misfi res (false data caused by rough road or other cause). • Total misfi res—a readout averaging the number of mis- fi res recorded during the last 200 crankshaft revolutions. • Misfi ring cylinder—shows the primary misfi ring cyl- inder (cylinder with the most misfi res) and the second- ary misfi ring cylinder (cylinder with the next highest number of misfi res) by cylinder number. • RPM at misfi re—shows the engine rpm when the com- puter detected a cylinder misfi re. Th is is handy for fur- ther scope diagnosis since you know the engine speed when the problem occurs. • Load at misfi re—gives the load (as a percentage) when the engine misfi re happened. Th is is usually informa- tion gathered from the manifold absolute pressure sen- sor, which measures engine load. Other misfi re data can also be produced depending on vehicle make, model, and year. Refer to the service manual for more information. Using an Oscilloscope An oscilloscope, or scope, is a piece of test equipment that displays voltages in relation to time. An oscilloscope may be part of a hand-held scan tool or multimeter or it may be a stand-alone unit, Figure 34-13. When connected to circuit voltage, the scope produces a line on a liquid crystal display. Th e line illustrates the various voltages present in the circuit over time. By comparing the scope pattern (line shape) to a known good pattern, the technician can deter- mine whether something is wrong in the circuit. Reading the Scope Screen Th e scope screen can give instructions, display volt- ages as a trace, or give other values as digital displays. Th e oscilloscope’s ability to draw a trace, or pattern of circuit voltages, in relation to very short time spans makes it useful for testing ignition and computer system performance. You should learn to recognize good scope patterns. Th en, you can easily detect scope patterns that indicate problems. Figure 34-13. This handheld scan tool also functions as an oscilloscope. It can display voltages in relation to time. Note the scope pattern on the screen. Handheld Scope Scope Screen