For 1990-2009 cars

Important: This check is used only to assist with idle HC/CO diagnostics.

• | Initial conditions:

- | The engine must reach normal operating temperature.

- | The air cleaner is installed.

- | All pipes and hoses of the air induction system are connected.

- | All vacuum lines are properly connected.

- | All fuel system wiring connections are fully plugged.

- | The transmission is in P (Park) or N (Neutral).

- | The HC/CO meter is calibrated and in hand.

• | Run the engine at 2500 RPM for approximately 2 minutes.

• | Insert the HC/CO meter testing probe into the intermediate pipe at least 40 cm (16 in).

| Important: Wait at least 1 minute before measuring to allow the concentration to stabilize. Complete the measuring within 3 minutes. If HC/CO concentration does not conform, see the following HC/CO diagnostic chart for possible causes.

• | Check the HC/CO concentration at idle.

#### HC/CO Diagnostic Chart

HC | CO | Symptoms | Causes
--- | --- | --- | ---
High | Low | Rough idle, very lean air-fuel ratio causing misfire | Faulty ignition:

• | Incorrect timing

• | Broken/faulty ignition components

• | Secondary wire/coil

• | Fouled, shorted, or improperly gapped plugs

Vacuum leak:

• | PCV system

• | Intake manifold/gasket

• | Throttle body/gasket

• | Brake booster/hose

High | Normal | Rough idle, low power/high oil consumption |

• | Leaky intake and/or exhaust valves

• | Faulty lifters

• | Leaky EGR plate gasket

• | Leaky head gasket or cracked head

• | Worn valve guide seals

• | Worn rings/valve guides

• | Faulty catalytic converter

High | Low | Rough idle (black smoke from exhaust), extremely rich air-fuel ratio | Faulty fuel system Faulty electronic fuel (system):

• | Faulty pressure regulator-high fuel pressure

• | Clogged fuel return line

• | Damaged intake air temperature sensor

• | Faulty PCM

• | Faulty fuel injector

• | Faulty throttle position sensor

• | Manifold absolute pressure (MAP) sensor

• | Damaged oxygen sensor

Normal | High | Poor fuel economy, poor idle, low power | Faulty fuel system:

• | Idle speed

• | Fuel injector leaking

• | PCV system clogged

• | Plugged air filter

• | Faulty pressure regulator-high fuel pressure

• | Faulty catalytic converter