Replaced egr value catalytic converter fails for excessive nox
SOURCE: 1994 cadillac sedan 4.9 engine will not pass smog.
The HC is high because the CO is high, CO is caused by to much fuel being delivered. You will want to check the Oxygen sensor and make sure it is functioning properly. You will want to check the fuel pressures, including rest pressure. We have seen leaking injectors cause this problem. You will want to make sure the thermostat is working correctly also.
First thing to do is check over the sensors that control the fuel mixture as it sounds like you're runnign too rich. This could be due to a faulty water temp sensor. If this is the problem then the ECU (Engine Control Unit / aka computer) will be leaving the car constantly 'on choke' and not reducing the fuelling as the engine warms - so unplug the sensor and clean the contacts on both sides with some switch / electrical cleaner (not WD40) and inspect the wiring for any signs of damage to the insulation especially where it runs over other components and through the bulkhead
If the engine is running well and the state of tune and ignition timing is good (15 degrees BTDC) then most likely the converter itself is bad. Usually NOx readings will go down as HC and CO readings go up, and as a result it is extremely rare for a car to fail an emissions test with all three things if the Catalytic Converter is working properly.
A bad oxygen sensor can also cause a failure similar to this, but will usually cause HC/CO readings to be high (but NOx readings would be low), or the opposite - high NOx but low HC/CO readings. The O2 sensor is located in the exhaust manifold just
before the bulge of the catylst and at the point where the 4 pipes join
together.
Unfortunately there is no good way to test the Cat. Highly specialized equipment is necessary to do this, however a temperature test might clue you in to one that is bad.
After driving the vehicle for several miles, immediately check the temperature of the converter at the very front and the very rear of the unit itself. The rear should be at least 100 degrees (F) hotter than the front. If the two are close to the same temperature then it is proof the converter isn't working very well.
I have to note though that temperature testing is far from conclusive. I've seen converters that pass test with flying colors but still ended up needing to be replaced.
Aftermarket converters tend to be less effective than factory converters because they are not built specifically for your vehicle. Instead, a universal unit is welded into some exhaust pipes so that it will fit. Generally this isn't an issue, but if you live in a 'green' state such as California where emissions standards are very strict, it could pose a problem.
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