ToddAndMargo

Hi All,

I keep pulling a code P0420 on my 2006 forester. I bought a
checker/resetter to clear the code. (Plan on fixing it in January -- a
new tax year.)

Here is the rub. I keep really close record on my gas mileage.
Take a look at this!

date miles gallons mpg
08/14/08 179.1 7.597 23.575
08/21/08 125.5 5.298 23.688
08/27/08 95.2 3.961 24.034
09/08/08 221.5 8.699 25.463
09/11/08 106.8 4.303 24.820
09/23/08 169.3 6.963 24.314

Code P0420 pulled on here
09/25/08 116.2 4.162 27.919

Code P0420 reset here
10/02/08 113.5 4.869 23.311


I am getting about 4 extra miles to the gallon with the P0420 code!
I reset it because I can not use my cruise control and risk unwanted
attention from the local constabulary.

What the heck? How does one get the extra 4 miles to the gallon
without the code?

Is there a way to get the cruise control to work with the check
engine light on?

-T

Shock_Resist

This might explain it: OBD-II Trouble Code: P0420 Catalyst System Efficiency Below Threshold (Bank 1)

bbottomley

It's a good thing if it gets you a free replacement cat right near the end of the 8/80 warranty.

__________________
2003 XS Premium MT - 2005 Impreza 2.5 RS MT

Andy_PHA

I don't know this for fact, but maybe the difference in the mileage isn't really the code, but the drop in MPG might be because you are resetting the ECU. As you may or may not know our ECU's are constantly learning our driving habits and adjust vehicle dynamics accordingly. This is just a wild guess, but I've noticed a similar problem with mine and that was my theory.

__________________
06 FXT Limited 4EAT 18G powered.

jpspecial5

I have been driving with a troublesome code of the same number, but have never lost cruise control.

ToddAndMargo

I don't thing I stated myself very clearly.

1) With CEL and Cruise blinking, due to code P0420, I
get 28 MPG. A 4 MPG improvement.

2) With no code, my MPG drops back to 24 MPG. Rats!

What I would like to achieve is 28 MPG and no code.

-T

thefoos

I have found this to be true as well. When I go through a series of reflashing, my mileage isn't as good. Once I settle on a tune and leave the ECU along for a while, I typically pick up 2-3 mpg.

__________________
Scott
04 MT FXT STG 2 5 spd
02 WRX STG 2 5 spd

massaa

When you reset the ECU, you also wipe the long-term fuel trim table. Fuel trim affects your mileage.

If you drive with the code long enough, the light goes out and cruise control starts working again - until the code reappears... Get the Tactrix cable and disable that specific code or switch back to your OEM exhaust.

ToddAndMargo

There have been no modifications to my exhaust. It is fully stock and
original

Okay. I drove all this week without the code. I hit 24.9 MPG. A
little better, but not 28 MGP with the code.

The previous owner was a business: a "Fleet Vehicle". If the employees
drove with a lead foot, I may be suffering from their education of my
"long-term fuel trim table". Also their elevation was 500'; my elevation
in 4,900'.

Question: when the code is triggered, does my ECU ignore my
long-term fuel trim table?

Question 2: would anything be hurt if I asked my mechanic do a full
reset on my ECU (and clear my long-term fuel trim table)?

Many thanks,
-T

thefoos

Question #1: No. Your ECU has trim ranges, and the reason you have the CEL is that your ECU thinks that it should trim further, but its not allowed. Therefore, the ECU thinks that something must be wrong. It still looks at your trims, you are just at one extreme most likely.

Question #2: All your mechanic will do is disconnect the battery and let it sit for a hour. You might as well do that yourself. That will reset your ECU and erase all of your trims. I will typically cycle the ignition to discharge the caps in the ECU to be sure you got a full reset.

__________________
Scott
04 MT FXT STG 2 5 spd
02 WRX STG 2 5 spd

ToddAndMargo

Thank you!

I think I will wait for the the P0420 to trigger again, then do the battery disconnect. That way
if the code disappears, I know I discharged the ECU correctly.

-T

Opie

Just a note...P0420 codes are typically caused by faulty catalytic converters, they are warrantied for 8year/80,000 miles. Don't pay to fix it if its still under that warranty.

__________________
FBO Member
'07 Forester 2.5XT, '05 Legacy 2.5GT

jpspecial5

2001 S auto. 96,000 K. miles.
This code can be triggered by mutliple causes, according to an Autozone readout.
My dealer pinned it as a failed knock sensor. 230 dollars and two tanks of gas after a new knock sensor, the check engine light went back on with that code.
This time they say a new catcon is in order. I said not at this time;
they turned off the light, expecting to see me again.
Two tanks of gas later, the check engine light has not been triggered.
Suppose it does pop back on. I will run it for a few weeks to see if it clears itself. If it does, does that eliminate the possibility of a failing catcon?
(In the car's lifetime, the code has cleared itself a couple of times.)

In other words, does a failing catcon keep that check engine light on, once the "black box" has been notified that the catcon has started to fail?

Thank you,
Joe

KeNd0

Wouldnt the gain in mileage be explained by the lack of cruise control ?

Graham

The ECU is not a "black box" in the normal usage of that term. A "black box" is a data recorder which will retain its contents despite it suffering moderate trauma such as loss of power or impact, and is generally used for purposes such as crash investigation.

The ECU has no permanent memory (other than the flash memory the programming is kept in). It's general memory is short term memory, which loses its contents when power is lost. A CEL occurs as a result of the ECU detecting sensor and other data outside certain limits. It will continue to monitor the data, and if the problem goes away (i.e. it is transient), the CEL will go off after a few drive cycles, and in time the code (now called a pending code) will be erased from the ECU's memory.

Resetting the ECU clears its memory (apart from the flash memory), which includes any CELs which have occurred recently, and also the adaptation it has made to your particular car (they all vary slightly) and the way it is driven.

So, the short answer to your question is "no", but if the same conditions continue, then the CEL will remain on.

__________________
07 Forester Sports XT 4EAT & 02 WRX wagon 5MT