High Hydrocarbon (HC) / Carbon Monoxide (CO) / Nitrogen Oxide (NO) Smog Failure

What causes high HC emissions?

Excessive HC results from ignition misfire or misfire due to excessively lean or rich air/fuel mixtures. 

  1. Engine Misfires
  2. Ignition System Failures
  3. Excessively lean air/fuel mixture
  4. Excessive EGR dilution
  5. Timing Issues
  6. Catalytic Converter
  7. Worn piston rings and cylinder walls
  8. Carbon deposits on intake valves

What causes high CO emissions?

High carbon monoxide levels are caused by anything that can cause a rich conditon.

  1. Excessive fuel pressure at the injector(s)
  2. Leaking fuel injector(s)
  3. Faulty O2 sensor(s)
  4. Defective fuel pressure regulator
  5. Defective EVAP system
  6. Plugged PCV valve or hose
  7. Catalytic converter

What causes high NO emissions?

NOx are produced from the reaction of nitrogen and oxygen gases in the air during combustion, especially at high temperatures & high pressures.

  1. Cooling system problems
  2. Fuel Mixture: excessively lean air/fuel
  3. EGR System Problems
  4. Overheating Issues
  5. Faulty O2 Sensor(s)
  6. Excess Carbon on Valves,
  7. Catalytic Converter
  8. Timing Issues
  9. Carbon deposits on intake valves


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