Problem With The Exhaust

[Kimmi]

I have a 1993 SC400 and having a problem with the exhaust numbers. CO = 0,9, HC= 140

The autoshop that have checked the car found that one of the oxygen sensor was malfunctioning. This has been replaced and now it seams that the lambda rates are correct, but unfortunately the numbers are as above. The explanation I have received is that the catalytic converters have to be replaced. This is something I want to be certain about before replacing them.

Here is what seems strange to me:

When the car is run at proximately 4000 rpm the numbers are very good. Then when the rpm goes down to 800-1000 the rates are still good, but after approximately 1 minute the exhaust numbers start to wonder up to CO=0,9 and HC=140. The thing I find strange is that if the catalytic converters where bad the rates aught to be bad the entire time.

Other info:

The car has recently been served with oil, sparkplugs (I don know if it makes a difference, but they are standard sparkplugs and not the platinum 6000 mile sparkplugs).

The motor works perfectly.

The cars check engine light when on earlier and gave the malfunction codes 21,25,26,51 so the autoshop changed one of the front oxygen sensors, so now the light is of. The two back sensors were changed in august. They have also checked the sensors, and the only thing that seams strange is the two back sensors are very active.

[bartkat]

You could try resetting the ECU by disconnecting the negative battery terminal for 20 minutes. Then start the car and drive about 10 miles of highway, then about 5 miles of town driving. Then test the emissions.