I have recently purchased a 97 Legacy L with 68K on the odo. A couple of days ago the CEL came on. I had it checked and it came up P0420 "Catalyst efficiency below threshold (bank 1)".

 

I'd like to generate a discussion specifically regarding this code, it's possible causes, suggested trouble shooting/solutions, and actual fixes or attempted fixes.

 

I've done a search on the site and it seems that others are experiencing the same thing. I know that modified engines/exhaust systems are prone to this but that should not be the case for stock vehicles. Modded vehicles bring there own particular problems so let's stick to "stock" solutions if possible. Many of us have to pass emissions so ignoring the code is not an option.

 

I'll try to compile here the information that I have seen (I have tried to give credit when possible). Let's discuss them and please add, correct or clarify.

 

Some notes.

 

A. The FSM indicates that this code could also be triggered by a fault with either O2 sensors circuit (upstream and downstream). If so, there should also have been another code indicating that. Moreover, even with no O2 sensors at fault, the P0420 code could be triggered by a leak in the exhaust system (air entering the exhaust system).

Finaly the FSM states that you have to inspect the inside of the cat converter before deciding it's bad. (posted by Frag 3-20-04)

 

B. AFAIK the replacement interval for Subaru O2 sensors is 100,000 miles. (posted by 99obw 12-24-03)

 

Suggested solutions.

 

1. Use correct grade of good quality gas.

 

2. Ensure correct spark plugs installed.

 

3. The factory manual says if there are no other trouble codes showing, to (1) check for leaks in the exhaust system, or (2) remove the front and rear catalysts and check for damage at the face of the catalysts. (posted by Harborseal55 2-1-04)

 

4. I would start by clearing the code. If the code returns then I would replace the rear sensor. (posted by 99obw 12-24-03)

 

5. Replace the catalytic converter.

 

Any other suggestions? What have you tried? What worked/ didn't work?

Ok so I have some questions right away.

 

"P0420 "Catalyst efficiency below threshold (bank 1)"" What is bank 1?

 

"Finaly the FSM states that you have to inspect the inside of the cat converter before deciding it's bad. (posted by Frag 3-20-04)" What do you look for in the converter to see if it's bad?

 

"1. Use correct grade of good quality gas." Would reformulated gas be a problem? The manual says it's ok but should you go with a higher grade like 89 octane?

 

"2. Ensure correct spark plugs installed." I heard that platinum plugs are bad and copper plugs should be used. Any truth to that?

 

"3. The factory manual says if there are no other trouble codes showing, to (1) check for leaks in the exhaust system, or (2) remove the front and rear catalysts and check for damage at the face of the catalysts. (posted by Harborseal55 2-1-04)" Face of the catalysts meaning gasket faces?

 

"4. I would start by clearing the code. If the code returns then I would replace the rear sensor. (posted by 99obw 12-24-03)" If the problem is intermittent, could there be some environmental factor? Is there a way to check if the sensor is bad?

This is Ford's explanation

 

 

http://www.fordscorpio.co.uk/catmonitor.htm

When's the last time you changed the fuel filter? A partially plugged filter can create a lean run condition and melt the catalyst over time.

 

Change it, clear the codes, and see if that works.

This weekend I had the code reset to see if it would come back right away. So far I have 70 miles on it with no light. I have been driving it in the same way as before, but no rain this time. Perhapse excessive amounts of rain over a couple of days is enough to keep the exhaust temperature down causing the fault.

 

I just bought this car so I have no idea when the fuel filter was changed last. That would be something to try prior to replacing O2 sensors. Is there a way to check if the sensors are operating properly?

Is there a way to check if the sensors are operating properly?

Scan the sensors with OBD2 scanner. Look for typical responses:

front-oscilation in closed mode, rear-oscilation with cold, flat line with hot cat.

p0420? that's sube's code to indicate time for a smoke break! seriously, i had the same code on my 99 leg l sedan 5-sp ej-22 75k miles. the code was set at least 2 times. one time was when the car was somewhat cold, i was accelerating uphill in 5th gear at low rpm's and lugging the engine. dealer replaced both cats under fed emission warranty, 6-yr 80k mile. free. it's obviously bad older cats as all sensors were fine. parts guy said it's happened with other late model cars and that he thinks the cat design has been changed or there was a bad batch or some such other excuse. i don't know if there has been a parts number revision. bill would have been about $1000 for parts alone. thankfully it was warranty covered. what's one to do? good luck.

hmm.. Interesting. I pulled that code last night. I use to run a ghetto-fabed intake and exhaust w/ 2 1/2 inch pipes. I now have stock intake back on.

 

Car is hitting 88k and I changed the fuel filter around 40k or so.

 

I cleared the code last night and I am hoping for the best. I will post any changes or corrects that I did.

PO420 alone is a bit of a scarry thing. like stated above it can be a false code if the sensors are bad or there is exhaust damage/leaks.

friendly j has a good point as to how O2 readings should appear. the rear o2 sensor should be just cruising along with little to no voltage change while the front O2 is oscilating like crazy.

oh yeah bank 1 question.

that is a J1930(OBDII) term used to distinguish banks like on a v6 or v8 there are 2 banks that are watched seperately. the bank with cyl.#1 is bank 1.

if it only monitors one bank(most 4 cyl. motors) it will simply call it bank 1

now you may see B1S1 or B1S2. B1 is bank 1 or B2 for bank 2. the S is for sensor the number folowing the S stand for before the cat(1) or after(2)

the second o2 is used primarily for catalist efficency while the first is used for fuel control.

I suggest clearing the code after O2 function is verifyed and see if it comes back. if it does well you may need a new cat.

Here's a quote from the Ford link given above:

 

"In 1996-on vehicles, the switch rate of the rear sensors is compared to the switch on the front. It follows that if the catalyst is efficient it will be removing HC and HO and the rear sensor will show a slow switch rate compared with the front. By dividing the number of downstream switches with those of the upstream gives a catalyst switch ratio, and a typical threshold is 75%, above which a DTC will be recorded." End quote.

 

So the front sensor must be switching at a rate substantially more vigorously than the rear sensor. The ECU measures the switch rate of both sensors and calculates a ratio. If the ratio falls below a pre-determined minimum, cat-efficiency code is displayed.

 

This switching ratio can be adversely affected by a slowing of the switch rate of the front sensor (leaky exhaust, contaminated front sensor), an increase in the switch rate of the rear sensor (contaminated cat), or both.

 

I'm sure there's a fair amount more to it, but that's the basics of the "efficiency" mechanism as I understand it.

1999 forester-s,

 

Just degreased & hosed off the engine.Started up fine,CEL came on.Drove the car 10 miles runs ok.How do I clear the CEL??

The easiest way I know is to take your car by Autozone, O'Reilley's, Advance Auto Parts, or the like and have them do a free diagnostic scan. (O'Reilley's won't do it, but they will "rent" the machine to you. Very easy to use.) Find out why the code came on (so you'll know if this is an issue you should be concerned with and for possible troubleshooting later) then clear the code with their machine. Hopefully you never see it come on again when you're driving. Good luck!

When's the last time you changed the fuel filter? A partially plugged filter can create a lean run condition and melt the catalyst over time.

 

Change it, clear the codes, and see if that works.

you will end up replacing the covertors eventually its expensive but doing sensors will not cure the problem i have worked in a dealership before i know so save the hassle of guessing replace them sorry to let you know