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Diagnosing Faulty Catalytic Converters

Created on 2019-02-01 by ipd Staff, Last Updated on 2019-04-04

*** Disclaimer; Direct from IPD’s Tech Tip archive!  This tech tip contains information from previous publications.  Products mentioned may not be available or the information may not be accurate due to changes in supply, manufacturing, or part number association.  Please contact IPD Customer Support if you have further questions info@ipdusa.com

We recently had several older Volvos (1981-1990) require new catalytic converters to pass local emissions testing. We've found that the new enhanced emission testing, now performed on a dyno in most states for cars 1981 and newer, is much tougher to pass. Often a new catalytic converter is the only way to get the car to pass. In the past it was possible to pass emissions test with a properly tuned car, however the new procedure tests under load and the catalytic converter must be working properly to reduce emissions to acceptable levels.

Converters function like an incinerator, burning excess emissions, which produces a lot of heat in the exhaust. The converter can handle this as long as there are "normal" amounts of pollutants in the exhaust. When too much unburned fuel or other contaminants enter the exhaust because of a misfiring spark plug, over-ly rich fuel mixture, or a leaky exhaust valve or head gasket, the converter's operating temperature can soar, causing internal meltdown. This results in partial or complete blockage.

Converters can also fail because of contamination. As a converter ages, the catalyst gets "tired" because of a gradual accumulation of contaminants on its surface. The process can be accelerated by oil burning (worn valve seals, guides and/or rings), internal coolant leaks (cracked head or block, or leaky head gasket), and sul-fur deposits normally found in gasoline.

As contaminants build up inside the converter, HC, CO and NOX emissions rise. On a late-model, well-tuned engine with a properly functioning converter, HC and CO emissions should be nearly zero. If an emis-sions test reveals higher than normal emissions, the converter may need attention. There's no way to rejuvenate a dead or damaged converter, so replacement is your only option.

DIAGNOSIS: Checking for converter contamination and or restriction

An emissions test is the best way to check for contamination by measuring the levels of HC and CO in the exhaust (and NOX in areas that test for this gas, too). An outright emissions failure and/or higher than normal HC, CO and NOX readings on a properly tuned engine in good condition would tell you the converter isn't doing much to reduce emissions. On 1996 and newer models with OBD II diagnostics, a contaminated con-verter will cause the Check Engine light to come on if the readings of the upstream and downstream 02 sen-sors reveal little or no change in oxygen levels as the exhaust passes through the converter. A PO402 trouble code indicates low converter efficiency.

Checking for restrictions is easier and doesn't require any special equipment. Suspect a restriction problem if your engine lacks power, has been using a lot of gas lately, or stalls after it starts and won't restart. A low intake vacuum reading also indicates excessive backpressure, which may be due to a plugged converter. If the vacuum reading drops and the engine stalls, the converter may be plugged. Note the reading at idle, then hold rpm at 2,500. The needle will drop when you first open the throttle, then stabilize. If the reading then starts to drop, backpressure is building up in the exhaust system. You can also attempt to measure exhaust backpressure directly. Remove the oxygen sensor and take your reading at its hole in the manifold or headpipe. Generally speaking, more than 1.25 psi of backpressure at idle, or more than 3 psi at 2,000 rpm tells you there's a block-age. A "thunk" test on the outside of the converter with a soft rubber mallet will tell you if the catalyst inside is loose. If you suspect a blockage, disconnect or remove the converter and look inside with a light. If you can't see through the honeycomb, the converter is obstructed and needs to be replaced.

Emissions, what to do if your car doesn't pass the test.

*Items are listed in order of probability.

If you have high CO and high HC, the engine is rich. You could have:

  • Mixture problems (rich)
  • Plugged air filter
  • Too high fuel pressure
  • Engine not fully warmed up or bad thermostat
  • Leaky fuel injector
  • Bad oxygen sensor
  • Contaminated oil
  • Bad cat

Low CO and High HC and the engine is lean, and misfiring, you could have:

(Note: If HC is high at idle and normal or low at cruise think vacuum leak, if it is high at both idle and cruise look elsewhere)

  • Bad ignition parts such as spark plugs, wires, cap and rotor
  • Timing advanced too far
  • Vacuum leaks
  • Bad injector seals on K-Jetronic
  • Worn engine parts like valve stem seals and piston rings that could cause oil burning
  • Too low a fuel pressure
  • Bad oxygen sensor
  • Bad cat

Note: If a misfire has damaged the cat replacing the cat without repairing the cause of the misfire will only destroy the new cat in short order, always fix the root cause first.

High NOX you could have:

  • Lean mixture
  • Timing too far advanced on 1988 and older cars (Note: on 1988 and older cars retarding the timing will reduce the NOX reading)

EGR system malfunctions like:

  • Bad valve
  • Broken vacuum line(s)
  • Plugged pipe
  • Bad vacuum controller
  • Engine / cat not fully warmed up
  • Bad cat

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