1994 Lexus LS 400 Fail emissions: High NO

Tiny
MR954
  • MEMBER
  • 1994 LEXUS LS 400
Engine Mechanical problem
1994 Lexus LS 400 V8 Automatic 172000 miles

Fails emisions test on 25 test for high NO.
25 test
reading allow
HC 11 103
CO .03 .57
NO 1570 712

15 test
reading allow
HC 18 107
CO .02 .59
NO 178 786

All maintenance is up to date: spark plugs, filters, etc. New pre-cat O2 sensors and cats are 2 yrs old. Cleaned out egr passages with seafoam twice. Have tried a used egr valve but same results. No vacuum leaks. Car runs good and no cel. Only thing bad is a slight valve cover leak that I think drips onto exhaust manifold and causes burning, but that shouldnt affect NO. New egr valve? Why NO is low on 15 test and so high on 25 test?
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Tuesday, November 24th, 2009 AT 5:25 PM

4 Replies

Tiny
KHLOW2008
  • EXPERT
Hi mr954,

Thank you for the donation.

The catalytic converter is responsible for the NO content and at low speed, the efficiency should be adequate. At higher speed, the efficiency reduces and this could be the cause of the problem. There is a possibility the cata or exhaust system is slightly plugged causing exhaust flow problem.

Was the catalytic converter used OEM or after market? After market parts usually does not last and can cause the problem though the CEL is not indicating.
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Tuesday, November 24th, 2009 AT 6:15 PM
Tiny
MR954
  • MEMBER
The cats were aftermarket ones made by catco. I actaully had the same problem last year with NO being high on the 25 test. Eventually it passed with a reading of 0 on 25 test and 300's on the 15 test. I was going to fail again and the test was stopped and we ran it one more time and just happened to pass with a reading of 0, which was weird going from 1400's to 0. Anyway to check tha cats for proper operation? Would the post-cat o2 sensors have an effect on the NO?
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Tuesday, November 24th, 2009 AT 9:51 PM
Tiny
KHLOW2008
  • EXPERT
The post cat O2 sensor would not have any effect on the NO but it would affect the CEL. It should be reading catalyst inefficiency but is not doing so.

From 1400 to 0 indicates an unstable catalytic converter and I believe that it is the cause of the problem.

For the year model I am unable to find any diagnostic procedures for the catalytic converter from our database.
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Wednesday, November 25th, 2009 AT 6:44 AM
Tiny
MR954
  • MEMBER
Put on two new cats made by Walker and still failed. Now i'm back to square one failing on both 15 mph and 25 mph tests. Nox again is above 1500. Hc and co are super low.
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Monday, March 1st, 2010 AT 2:44 PM

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