P0420 code

[edrach]

Anything to worry about. I get this every 4 to 6 weeks on my '97 Impreza. I clear it and it takes another 4 to 6 weeks to come back again. It's an O2 sensor code. Should I be worried?

[johnceggleston]

Anything to worry about. I get this every 4 to 6 weeks on my '97 Impreza. I clear it and it takes another 4 to 6 weeks to come back again. It's an O2 sensor code. Should I be worried?

 

if you search p0420 and read all the threads you will learn that there 2 schools of thought for correcting this code.

 

1

a new "subaru only" front o2 sensor.

 

2

new cats

 

what ever the cure, it is probably causing poor gas mileage and emissions that are not to spec and maybe? aging your cats. depending on the size of your wallet, where you live inspection wise and and your concern for the environment you can live with this a good while. but a new o2 will not break the bank and has a good chance of correcting the problem.

[edrach]

I read the other post on this and I'm still not clear whether it's the front sensor or the rear (my code reader says rear). For the record, I'm up to 179K on the engine/car.

[bgambino]

if you search p0420 and read all the threads you will learn that there 2 schools of thought for correcting this code.

 

1

a new "subaru only" front o2 sensor.

 

2

new cats

 

what ever the cure, it is probably causing poor gas mileage and emissions that are not to spec and maybe? aging your cats. depending on the size of your wallet, where you live inspection wise and and your concern for the environment you can live with this a good while. but a new o2 will not break the bank and has a good chance of correcting the problem.

 

of course there is one more alternative....I put two anti foulers in my back cat and no more annoying check engine light and P0420 code

[johnceggleston]

I read the other post on this and I'm still not clear whether it's the front sensor or the rear (my code reader says rear). For the record, I'm up to 179K on the engine/car.

 

most of the folks who take this to the dealer are told they need new cats, lots of folks here replace the front o2.

 

what i have read here before is that the front sensor is the working sensor and the rear is just a monitor. i don't know how accurate that statement is. but your best shot is a new subaru front sensor.

 

where's nipper when i need him?

[naru]

I read the other post on this and I'm still not clear whether it's the front sensor or the rear (my code reader says rear). For the record, I'm up to 179K on the engine/car.

 

Either or neither.

 

p0420 is "Catalyst System Efficency Below Threshold"

Computer sets this code by comparing the upstream and downstream sensor output voltages.

 

In a perfect world,a faulty sensor should set a sensor specific code before the p0420.

 

I would want to see the sensor voltages on a scan tool before replacing the cat(or either sensor).

 

Faulty CTS could trigger this code by telling the computer to do the comparision too soon.

 

Exhaust leaks could be a factor.

 

Personally,I wouldn`t be too worried if the code takes that long to reset.

 

I don`t buy the "Subaru only" theme.A sensor is a sensor.Subaru doesn`t make them.

Edited February 16, 2010 by naru

[Durania]

I too tried the anti-fouler trick to fix my P0420. I was getting it every few days and it would go off for a few. I had my buddy install the anti-fouler when he had it doing my head gaskets. He said he was only able to get one fouler on there instead of the needed two. Now my light is on a lot more.

[eppoh]

FWIW, I had this same code a couple months ago. At my son's suggestion, I ran a can of B-12 Chemtool through the intake, by way of the brake vacuum line. It smoked a little an the cat got so hot it smoked too.

 

I think there may have been some oil or soot residue in the cat that burned out.

 

Reset the code, and it has never come back. Car has 160K on it.