General Tech Help Good at troubleshooting? Have a non specific issue? Discuss general tech topics here.

Catalyst Efficiency Below Threshold--

  #1  
Old 05-15-2008, 09:38 PM
Thread Starter
Join Date: May 2008
Location:
Posts: 2
Default Catalyst Efficiency Below Threshold--

My wife drives the 98 Accord. Approx 140,000 miles on it. Just replaced upstream O2 sensor and had the cat replaced (aftermarket Fits-all) at 99,000 miles. Check Engine Light keeps coming back on after resetting shows the Catalyst Efficiency Below Threshold. Removed and cleaned EGR valve as per dealership. Have performed Injection cleaning service w/ intake cleaning and tank additive. What next. Car runs ok but only getting 28 mpg on hwy. I did timing belt at 110,000 miles and replace oe NGK plugs with NGK iridium plugs. Could that be the problem???? Help before I start throwing money at it. I just think the gas mileage should be better than 28. My 88 with 335,000 miles gets 31 mpg (it is a 5 speed though). Thanks for any help. Mike
 
  #2  
Old 05-15-2008, 10:00 PM
Member
Join Date: Nov 2004
Location: madison WI
Posts: 214
Default RE: Catalyst Efficiency Below Threshold--

probally a bad aftermarket cat, just buy an 02 SIM and forget about it...
 
  #3  
Old 05-15-2008, 10:41 PM
JimBlake's Avatar
Super Moderator
Join Date: May 2006
Location: Wisconsin
Posts: 17,368
Default RE: Catalyst Efficiency Below Threshold--

The '98 is lots heavier than the '88, besides the 5-speed. Between those2 itseems like enough to explain 31 mpg vs. 28.
 
  #4  
Old 05-16-2008, 07:51 AM
TexasHonda's Avatar
Super Moderator : And A Texan
Join Date: Feb 2007
Location: Katy, TX
Posts: 9,649
Default RE: Catalyst Efficiency Below Threshold--

Exhaust system leaks can cause P0420 "Catalyst efficiency" code to occur. Check exhaust manifold and connections for cracks or leaks.

good luck
 
  #5  
Old 05-16-2008, 04:43 PM
Thread Starter
Join Date: May 2008
Location:
Posts: 2
Default RE: Catalyst Efficiency Below Threshold--

poff----not sure the accronym SIM for the O2 sensor...please explain. Thanks for the info everyone. Exhaust has puffing noise coming from the pipe near rear of car...would a leak past the cat cause the DTC?????Thanks.
 
  #6  
Old 05-16-2008, 05:15 PM
Super Moderator
Join Date: Sep 2006
Location: Houston, TX
Posts: 13,034
Default RE: Catalyst Efficiency Below Threshold--

Which O2 sensor did you replace? The O2 sensor after the catalytic converter is the one that monitors the cat. I think that the connector goes through the floorboar and is under the drivers seat. Make sure that connector is clean and secure.

The SIM means that you can buy an O2 simulator that will put out the proper signal to ECU and fool it into thinking that the catalytic converter is working properly. That second O2 sensor montioring the catalytic converter has nothing to do with controling the air/fuel ratio. Its only job is to monitor the catalytic converter. Adding a SIM will not change the car's performance.

As for gas mileage, here is what a quick google search came up with for the 98 accord. You gas mileage is normal.
MPG (city) [align=center] [align=center]21[/align] [/align] MPG (highway) [align=center] [align=center]28[/align] [/align] MPG (combined) [align=center]24[/align]
 
  #7  
Old 05-16-2008, 05:42 PM
Member
Join Date: Nov 2004
Location: madison WI
Posts: 214
Default RE: Catalyst Efficiency Below Threshold--

an o2 sim is short for o2 simulator, simulates a 14.7 stociometric reading to the computer so it thinks the system is fine, thus no check engine light.
 
  #8  
Old 05-16-2008, 07:11 PM
Join Date: May 2008
Location:
Posts: 59
Default RE: Catalyst Efficiency Below Threshold--

At our shop, Accurate Automotive near Nashville, we tried using aftermarket CATS for awhile....They do not hold up longer thana year, before you get a another catylist insufficient code...Some customers came back after only a few days (the same scenario as you have).....

Want to do yourself a favor? It's gonna hurt, but just install a Genuine Honda CAT and be done with it. If you love your Accord and plan to keep it, you probably won't have to replace it again for another 10 years.

One reason yourfuel mileage is going down is because the check engine light is on and the vehicle is not running in open loop which is the most efficient, where all sensors are sendng information to the ECU for the most optimal fuel ratio. Instead, the check engine light is on, so the ECU is running in closed loop, which is a pre-programmed, one size fits all ratio where sensor inputs are not calculated into fuel ratio.

Bite the bullet and install a Genuine Honda CAT and be done with......

H O N D A....we make it simple

Ed Brian
Accurate Automotive, Inc.
www.accuratecars.com


 
  #9  
Old 05-16-2008, 07:42 PM
finch13's Avatar
Been Around A Long Time Member
Join Date: Jun 2007
Location: St. Paul, MN
Posts: 7,381
Default RE: Catalyst Efficiency Below Threshold--


ORIGINAL: hondaed

One reason your fuel mileage is going down is because the check engine light is on and the vehicle is not running in open loop which is the most efficient, where all sensors are sendng information to the ECU for the most optimal fuel ratio. Instead, the check engine light is on, so the ECU is running in closed loop, which is a pre-programmed, one size fits all ratio where sensor inputs are not calculated into fuel ratio.
I'm going to have to argue that one. My 1999 has had the P0420 Cat Below Threshold code since 2001, that's 7 years with the code and has not suffered a loss in mileage. In fact, the other week I got 34 MPG.


If the sim doesn't affect performance I might just go with that, is it just the extended bung that pulls the O2 sensor out of the pipe or is it an electronic gizmo? Where can I get one?
 
  #10  
Old 05-16-2008, 09:36 PM
TexasHonda's Avatar
Super Moderator : And A Texan
Join Date: Feb 2007
Location: Katy, TX
Posts: 9,649
Default RE: Catalyst Efficiency Below Threshold--

http://www.scoobymods.com/forums/showthread.php?t=5468

Above link describes how to install the spacer to withdraw the rear O2 sensor slightly out of the exhaust stream and eliminate P0420. I used this on a Toyota Celica and it's worked for over 2 yrs now.

good luck
 

Thread Tools
Search this Thread
Quick Reply: Catalyst Efficiency Below Threshold--


Advertising

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service

© 2019 MH Sub I, LLC dba Internet Brands

We are a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for us to earn fees by linking to Amazon.com and affiliated sites.