2000 Legacy L: P0420 Sensor Bank 1?

[Ravenwoods]

My daughter's 2000 Legacy has the Check Engine Light on and the code is P0420. Something about Bank 1. Automatic Transmission. About 180,000 miles. 

So is the code referring to the Oxygen sensors on the Catalytic Convertor? One upstream and one downstream from the catalytic converter? Or is this somewhere else? The car is running normally with good gas mileage.

[1 Lucky Texan]

6 cyl or 4cyl?

scan for and make note of any other codes including 'pending' codes. Clear the codes and see how quickly the P0420 comes back and any other codes as well.

many people jump to changing the cat conv (and of course they do go bad) but, that code can be caused by exhaust leaks, vacuum leaks or bad wiring. The code on the 4 cylinder cars is only a 'nanny' code intending to monitor cat efficiency but is NOT used to alter any air:fuel ratios or other operating parameters.

Edited April 27, 2023 by 1 Lucky Texan

[Ferret54]

When we had this code on our daughters 2001 Outback ( 2.5L ), it was 10 years old. No exhaust leaks. Tried rear Oxygen sensor, still P0420. So I purchased an extender. Had to drill out the center for the Oxygen sensor to fit, then installed it. Takes rear sensor out of the flow. No more P0420. It was still on the car when we sold it 2 years later.

[lmdew]

Been clearing the code for years on various Subaru cars.

 

[Ravenwoods]

6 hours ago, Ferret54 said:

When we had this code on our daughters 2001 Outback ( 2.5L ), it was 10 years old. No exhaust leaks. Tried rear Oxygen sensor, still P0420. So I purchased an extender. Had to drill out the center for the Oxygen sensor to fit, then installed it. Takes rear sensor out of the flow. No more P0420. It was still on the car when we sold it 2 years later.

I have no idea what an extender is.

[Ravenwoods]

My daughter drives the car only for short trips. I suspect some of the problem is she never takes long road trips. I borrowed the car last month to take a 140 mile trip to test drive the car. No check engine light the whole day. But as soon as she started driving it again the light has been on the whole time.

[lmdew]

Hopefully Dad has an OBDII reader for her and has taught her how to use it.  Easy to clear.

[Daskuppler]

When this code popped up on my 2002 Impreza it was head gaskets. I had some.other driveability issues indicating the failure though

[Ravenwoods]

On 4/27/2023 at 9:50 AM, 1 Lucky Texan said:

6 cyl or 4cyl?

scan for and make note of any other codes including 'pending' codes. Clear the codes and see how quickly the P0420 comes back and any other codes as well.

many people jump to changing the cat conv (and of course they do go bad) but, that code can be caused by exhaust leaks, vacuum leaks or bad wiring. The code on the 4 cylinder cars is only a 'nanny' code intending to monitor cat efficiency but is NOT used to alter any air:fuel ratios or other operating parameters.

Its a 4 cylinder 2.5 liter.

[1 Lucky Texan]

IF fuel trims and knock sensor and vacuum, and performance, etc. are all OK, you could ignore the code. Problem is, many locales would never allow the car to pass emissions/safety test, AND, the cel would be on all the time -  masking any new problem detection.

just eliminate every other possible cause before jumping to a new after-cat sensor or new cat. converter.

Edited April 29, 2023 by 1 Lucky Texan

[Ravenwoods]

On 4/28/2023 at 2:03 AM, lmdew said:

Hopefully Dad has an OBDII reader for her and has taught her how to use it.  Easy to clear.

So sounds like the best approach is to ignore it?

[forester2002s]

I get the P0420 on my 2002 EJ251 every few months, and then I delete the code.

My car normally just does local runs. But once in a while, I go on the freeway at high speed; if it's a hot day, the CEL comes on and it's a P0420 code. So I delete it.

Been doing this for years. The engine runs beautifully!

[idosubaru]

On 4/29/2023 at 5:02 PM, Ravenwoods said:

So sounds like the best approach is to ignore it?

Yea. It’s 100% benign for pre 2005 models. The ECU doesn’t use that data for fuel trim. It’s not indicative of anything but emissions. 

the biggest issue is you can’t tell when the code trips again.

It’s routinely ignored in states that don’t check the check engine light for emissions or inspections for 100s of thousands of miles. 

Install an extender $10 on eBay and the code will go away. It’s just a spacer - remove rear O2 sensor, install spacer and reinstall O2 sensors into the spacer. It just makes the rear O2 sensor sit a little off the exhaust stream which is enough to trick 90% of them to not trip the code. 

repair usually includes replacing the converter. Aftermarkets sick and can cause the same issue in 1 month or 2 years. Subaru converters are $500+. So it’s not economical to repair if it’s not needed. 

it can often be caused also (conflating causes with an tires converter) due to an exhaust leak, fuel trim, vacuum leak, etc. but it’s nearly impossible to track down unless you’re a signals wizard. Most of the times the repair will be installing a new converter and making sure the engines working properly.