Catalytic Converter



Causes Of Converter Failures

  • Gets too hot due to unburned fuel that lights off inside the converter causing fouling, clogging, melt-down and breakage of the ceramic substrate.
  •  A bad oxygen sensor or coolant sensor may be giving the computer bogus information. A sluggish or dead O2 sensor will make the computer think the exhaust is running lean, so the computer will try to compensate by making the fuel mixture rich. A coolant sensor that always indicates a cold engine will also keep the system in open loop, which means a steady diet of excess fuel. But it might not be the sensor’s fault. A thermostat that’s stuck open or is too cold for the application can prevent the engine from reaching its normal operating temperature. So if your converter has failed and needs to be replaced, the engine should be diagnosed for any underlying problems before the new converter is installed.
  • Another cause of converter clogging and contamination is excessive oil consumption. Worn valve guides or seals can allow oil to be sucked into the engine’s combustion chambers. The same goes for worn or damaged rings or cylinders. Oil can form a great deal of carbon, and metals present in the oil can contaminate the catalyst. A compression check or leak-down test will tell you if the rings are leaking, while a fluttering vacuum gauge needle will help you identify worn valve guides.



Catalytic Efficiency Below Threshold
  • Bad catalytic converter
  • Wrong input from rear O2 sensors
    • Bad rear O2 sensor
    • Poor ground for the rear O2 sensor can cause sensors to work erratically and/or lean away from the 0.45V
    • Aftermarket sensor
  • Exhaust Leaks
    • Negative exhaust pulses pull in outside air and it skews the O2 readings





Catalytic converter Test

Vacuum Test
  • Connect the vacuum gauge to a source of intake vacuum on the intake manifold, carburetor or throttle body. Note the reading at idle, then raise and hold engine speed at 2,500. The needle will drop when you first open the throttle, but should then rise and stabilize as the engine rpms rise and the load is reduced.  If the vacuum reading starts to drop with steady high rpm, pressure may be backing up in the exhaust system. No manifold vacuum means that the engine can’t draw any more air/fuel charge.
  • If your engine has air injection, disconnect the check valve from the distribution manifold, and connect a low pressure gauge. Hold the engine’s speed at 2,000 rpm and watch the gauge. The desired pressure reading will be less than 1.25 psi (8.61 kPa). A very bad restriction will give a reading of over 2.75 psi (18.96 kPa).
Inlet-Outlet Temperature Difference
  • A good converter will usually run 100 degrees F (38°C) hotter at its outlet than its inlet. Little or no temperature change would indicate low efficiency, or a problem with the converter’s air supply. Converters need supplemental oxygen in the exhaust to re-burn pollutants, so if the air injection system or aspirator valve isn’t doing its job the converter can’t do its job either.
Rattle Test
  • Smack the converter with a rubber mallet. If the converter rattles, it needs to be replaced and there is no need to do other testing. A rattle indicates loose catalyst substrate, which will soon rattle into small pieces. This could be called a test, but it is generally not used to determine if a catalyst is good.
Observe O2 Readings
  • Engine should be in the closed loop. If catalytic is broken inside and passes gases through,  downstream O2 sensor will give up and down readings, instead of being steady close to 0.45V. It will roughly mimic readings of the upstream O2 sensor. Perform this test both with low and high engine speed.
    • Over time, you will come to learn the differences between a good, bad and even a marginally performing catalyst, by observing the downstream O2 sensor behavior through varying engine speeds. Often you will see a catalyst take extended preconditioning periods before it achieves “light off”. Other times you will see a catalyst that copes well with gas flow at low engine speeds, but starts to show minor symptoms as the gas flow rate increases at a higher RPM.
Idle Catalyst Monitoring (ICM) Test
  • ICM is another method of testing the catalytic converter’s efficiency (or Oxygen Storage Capacity, sometimes called OSC). The Idle Catalyst Monitoring Test is performed at idle instead of at highway speeds as in the past. For the PCM to run this test, all relevant enable criteria must be satisfied. The PCM will then drive the fuel mixture from lean to rich (or rich to lean) while monitoring the pre- and post-catalyst HO2S. High catalytic converter efficiency is recognized when the post converter HO2S signal does not closely follow the voltage toggle of the pre-catalyst HO2S. If both HO2S’s react quickly to the PCM’s change in fuel mixture, the PCM concludes that the catalytic converter’s efficiency is low (has low OSC).
  • OBD systems that use the Idle Catalyst Monitor have the capability of running the diagnostic while the vehicle is being serviced. The purpose of this technician-directed test is to (1) determine the catalyst performance following a catalyst-damaging event, or (2) exercise the diagnostic until the catalyst diagnostic’s I/M flag is set for vehicles that have not set this flag.


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