Troubleshooting If the HC/CO concentration does not comply with regulations, perform troubleshooting in the order given below. 1. Check oxygen sensor operation. (See page FI-139) 2. See the table below for possible causes, and then inspect and correct the applicable causes if necessary. HC | CO | Symptoms | Causes High | Normal | Rough idle | 1. Faulty ignition: • Incorrect timing • Fouled, shorted or improperly gapped plugs • Damaged or loose high-tension cords • Cracked distributor cap 2. Incorrect valve clearance 3. Leaky EGR valve 4. Leaky intake and exhaust valves 5. Leaky cylinder High | Low | Rough idle | 1. Vacuum leak: (Fluctuating HC reading) | • Vacuum hose • EGR valve • Intake manifold (Air intake chamber) • Throttle body • ISC valve • Brake booster line 2. Lean mixture causing misfire High | High | Rough idle | 1. Restricted air filter (Black smoke from exhaust) | 2. Faulty EFI system: • Faulty pressure regulator • Clogged fuel return line • Faulty air flow meter • Defective water temp. sensor • Defective air temp. sensor • Faulty ECU • Faulty injector • Faulty cold start injector • Faulty throttle position sensor