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

 

probally a bad aftermarket cat, just buy an 02 SIM and forget about it...

 

The '98 is lots heavier than the '88, besides the 5-speed. Between those2 itseems like enough to explain 31 mpg vs. 28.

 

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

good luck

 

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.

 

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]

 

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.

 

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

 

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?

 

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