The first thing you actually need to do is diagnose and repair trouble code PO172. That problem is most likely causing the cat efficiency code. The control module continuously checks three-way catalytic converter (TWC) efficiency by comparing signals from the rear heated oxygen sensor (HO2S) with the front heated oxygen sensor (HO2S).
Possible Source
misfiring
air leakage in the intake system
air leakage in the exhaust system
faulty fuel pressure
uneven compression
defective three-way catalytic converter (TWC).
Now for a description of PO172
Condition
The control module receives information from the heated oxygen sensor (HO2S) about the fuel/air mixture when idling at both the lower and upper part-load range. If the fuel/air mixture deviates from Lambda=1, the short-term fuel trim will compensate for this by adjusting the injection time so that Lambda=1 is achieved. When the short-term fuel trim makes an adjustment, the integrator median must be adjusted by the long-term trim. Diagnostic trouble code (DTC) ECM-262A is stored when the long-term trim must be adjusted to almost maximum in the upper part load area
Substitute value
none
Possible source
Upper limit:
intake air leakage
exhaust system air leakage
low fuel pressure
defective heated oxygen sensor (HO2S)
So first thing to check is for a intake leak. If you have it smoked you should be able to find the leak. If no leaks are found then this one I rarely see anyone do and it does effect the oxygen sensor but if you see possible sources for both codes you will see check the exhaust system for leaks. To check this they would need to pressurize the exhuast system and spray it down with some soapy water. Very slow forming bubbles is normal but if it starts quick and fast then you have a leak. Smoking the exhaust wil not work because there is not enough pressure. If that is OK then parameters for the engine control module need to be looked at. What they need to look at is mass air flow signal, mass air flow correction factor, and air leakage through the throttle blade. Looking at this numbers they can determine if the throttle is dirty. If it is, then the pcv system needs to be checked to see if it is plugged up. If it is then it needs to be cleaned out, replace the oil trap and clean out the throttle. You may also need to have the engine control modules software updated with the current software if one is available. A plugged pcv system and dirty throttle will effect what the oxygen sensors read cusing some contradiction into thinking there is a problem with the catalytic converter. Those cats are very expensive and if the cat has failed and you have problems like the ones mentioned earlier, then the new one will fail soon as well because you did not repair the root problem for the failure. If you trully believe your mechanic went through all this effort and the diagnosis is good let me know and I will give you directions to replacing the catalytic converter.
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Saturday, June 19th, 2010 AT 1:03 AM
