Symptom Related Diagnostic Procedures

If the Carbon Monoxide (CO) or Hydrocarbons (HC) concentration does not comply with regulations, perform troubleshooting in the order given below.

1. Check heated oxygen sensor operation.
2. If heated oxygen sensor tests OK, inspect systems as indicated below.

CO normal / HC high with rough idle
Inspect the following:
- Faulty ignition, i.e..; incorrect timing, fouled / shorted or improperly gapped spark plugs, open or crossed high tension cords, cracked distributor cap.
- Incorrect valve clearance.
- Leaky Exhaust Gas Recirculation (EGR) valve.
- Leaky intake and exhaust valves.
- Leaky cylinder.

CO low / HC high with rough idle and fluctuating HC reading.
Inspect the following:
- Vacuum leaks, i.e..; Positive Crankcase Ventilation (PCV) hose, EGR valve, intake manifold, air intake chamber, throttle body, brake booster line.
- Lean mixture causing misfire.

CO high / HC high with rough idle and black smoke from exhaust.
Inspect the following:
- Restricted air filter.
- Faulty fuel systems, i.e..; faulty pressure regulator, clogged fuel return line, defective Engine Coolant Temperature sensor, defective intake air temperature sensor, faulty Engine Control Module, faulty injector, faulty cold start injector, faulty throttle position sensor, faulty manifold absolute pressure sensor.