For 1990-2009 cars

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

1. Check for the following initial conditions:

• | Engine must reach normal operating temperature.

• | Air cleaner installed.

• | All pipes and hoses of air induction system connected.

• | All vacuum lines properly connected.

• | All fuel system wiring connections fully plugged.

• | Transmission in P (Park) or N (Neutral) range.

• | Tachometer and HC/CO meter calibrated and in hand.

2. Run engine at 2500 RPM for approximately 2 minutes.
3. Insert HC/CO meter testing probe into 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 the HC/CO concentration does not conform, see the following HC/CO diagnostic chart for possible causes.

4. Check HC/CO concentration at idle.

#### General Diagnostic Information 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

• | Secondary Wire/Coil

• | Fouled, Shorted or Improperly Gapped Plugs

Vacuum Leak

• | PCV Hose

• | Intake Manifold/Gasket

• | Throttle Body/Gasket

• | Brake Booster/Hose

• | Engine Pre-conditioning

High | Normal | Rough Idle, Low Power/High Oil Consumption |

• | Leaky Intake and/or Exhaust Valves

• | Faulty Lifters

• | Leaky EGR Valves

• | Leaky Head Gasket or Cracked Head

• | Worn Valve Guide Seals

• | Worn Rings/Valve Guides

• | Faulty Catalytic Converter

• | Engine Pre-conditioning

High | Low | Rough Idle (Black Smoke From Exhaust), Extremely Rich Air-Fuel Ratio | Faulty Fuel System and/or Faulty Electronic Fuel (System)

• | Faulty Pressure Regulator-High Pressure Fuel

• | Clogged Fuel Return Line

• | Damaged Intake Air Temperature Sensor

• | Faulty PCM

• | Faulty Fuel Injector

• | Faulty Throttle Position Sensor

• | MAP (Manifold Absolute Pressure) Sensor

• | Restricted PCV Valve

• | 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

• | Engine Pre-conditioning