322 Auto Engine Repair Copyright by Goodheart-Willcox Co., Inc. Figure 15-25. This modern catalytic converter system has three three-way converters and three oxygen sensors to monitor and treat exhaust flow out of the engine. (Nissan) Drive the vehicle to bring it to operating temperature before testing. Never test a vehicle with a cold engine or inac- curate readings will result. Warm the analyzer as described by the manufacturer. Then, zero and calibrate the analyzer while sampling clean air. Generally, take two readings, one with the vehicle at idle and a second at approximately 2500 rpm. Compare the analyzer readings with specifications. When testing some electronic (computer) fuel injection systems without a load, only readings at idle will be accurate. A dynamometer must be used to load the engine to simulate actual driving conditions. HC Readings An exhaust analyzer measures hydrocarbons (HC) in parts per million (ppm) by volume. An analyzer ading of 10 ppm means there are 10 parts of HC for every mil- lion parts of exhaust gas. If the exhaust analyzer reading is higher than specification, the vehicle’s HC emissions (unburned fuel) are too high. An adjustment or repair is needed. Higher-than-normal HC readings can be caused by: • Rich or lean air-fuel mixture. Leaking fuel injector, faulty pressure regulator, or improper fuel pressure. • Improper ignition timing. Distributor, computer, or adjustment problem. • Engine problems. Blowby, worn rings, burned valve, or blown head gasket. • Faulty emission control system. PCV system problem, bad catalytic converter, faulty EGR valve, or evaporative control system problem. • Ignition system troubles. Fouled spark plug, cracked distributor cap, or open spark plug wire. • Computer control system problems. Defective input sensor, output actuator, or ECM. Note: Always refer to the emission control sticker in the engine compartment or service manual for emission level specifications. Values vary from model year to model year. CO Readings An exhaust analyzer measures carbon monoxide (CO) in percentage by volume. A reading of 1% means that 1% of the engine exhaust is CO. High CO is basically caused