2002, P0420 and poor fuel efficiency?

[googabee]

Hi gang, I've a 2002 outback 2.5 with 176k on it. Purchased last June. Lately I've had the p0420 'catalyst performing below threshold' code come up repeatedly. Seems to happen after about 10 minutes of highway driving.

I had been getting around 26mpg mostly highway driving, but my last few tanks have gotten worse, with the last one only getting about 22mpg. First mech. who reset the code said his customers had had some luck just using premium fuel, which I haven't done. Another said probably replace the cat. Both independent shops. I have run a bottle of chevron techron thru it, and the auto store guy said I should replace the downstream O2 sensor before the cat converter.

I just ordered a Dayco timing belt/water pump kit, as I don't know when it was replaced last, and I'd like to have the shop fix the p0420/fuel consumption issue as affordably as possible while it's there. Unfortunately I'm not much of a DIYer.

I plan to go with a Bosch Premium sensor 'with OE smartlink', which the online store says is an exact fit, and I figure on a new PCV valve and air filter as well.

Is there anything else you might suggest I have done short of a new cat? Does the fact that I've seen a significant decrease in fuel efficiency over the last 1000 or so miles point to a more serious problem or probable cat converter failure?

This is my first Subaru and I really dig it, but need to get this problem behind me!

Thanks for any help, and have a great new year!

[SE95]

Keep in mind your local gas stations had probably switched over to winter blend fuel within the last few months, which might partially explain your drop in fuel economy. In January of last year I drove my Outback to Toledo on winter blend fuel and snow tires, I did just under 21mpg, all highway blast at right around the speed limit. Same trip in June on my Yokohama all seasons on summer blend fuel and my mpg was up to almost 27 mpg.

[rasterman]

Do a bit more reading- P0420 is a very common code, has multiple causes and each cause has its own solution.

Very few of those solutions involve replacing the cat. They just don't go bad very often.

O2 sensors fail much more often than cats, but keep in mind there is a difference between a mechanic and the parts store guy: one needs to fix cars for his paycheck, the other only needs to sell parts.

I'm not suggesting that you're being given bad advice, as there is a real possibility that the O2 sensor is causing this problem and a simple replacement will get you good as new. Rather I'm just reminding you that this is the parts store guy's bet, and it's a bet with your money.

You may not be in a great position to do further diagnostics yourself, but a bit more reading on the issue may well save you a few bucks all the same:

This thread is a great starting point for P0420 education.

[grossgary]

Converter it fine. Period.

Good point above - what kind of driving - it might just be winter fuel and some minor performance lagging?

Tune up - spark plugs, wires, and air filter? All really easy and cheap and should be done anyway. Start there. Use NGK OEM plugs and Subaru wires or at least high quality wires, that engine is not very forgiving with cheap ones.

If you're unsure about tune up items I would start there.

You are wise to replace the belt and all the pulleys/tensioner, interference engine so if the belt breaks or more commonly a pulley fails - you'll have bent valves on that motor.

Brakes dragging hurts mileage and is common in the rust belt. The caliper slide pins can seize not allowing the pad to pull off the rotor fully. On those 00-04 era stuff one of the front pins and back pins has a stupid bushing in it that swells and hangs the pads that way. Often happens right after a brake job too. I remove all the bushings and throw them away, they are not needed and never used on prior models and cause too many issues - we don't need any help with seizing caliper pins in the rust belt, they do it all the time anyway!!! I've seen 2-3 mpg improvements after a brake job. Needs to be done right though - remove slides, clean them, and regrease. At this age in the rust belt sometimes it's not bad to replace the pad clips and boots for the pins either.

RockAuto Auto Parts has awesome prices, though make certain you get the right parts. i just bought front ceramic pads for your vehicle for like $23 and the hardware - pad clips and boots for just a few dollars. Great deal and half or better the price from the local parts places. You can always find a minimum %5 discount code too online.

The 0420 code is thrown by the rear O2 sensor. But replacing that sensor will not gain you any mileage - because that signal is not used by the ECU for controlling the engine.

Front O2 sensor may be lazy - effecting gas mileage and the catalyst efficiency comparison algorithms.

The 0420 code is benign - you can drive with that for the life of the vehicle with no harm to the vehicle (older Subaru's don't even have that code or a rear O2 sensor and none of them are failing because of it). So i would focus on the gas mileage issue first - and you may take care of the 0420 code as well.

If you eventually need to diagnose the 0420, come back and let us know. In the end there's a $5 extender fix that works. I just sent one to a guy I sold a car too that lives far away and his mechanic installed it in his Subaru. Very common issue to other manufacturers as well, friends honda has it now.

[cardoc]

You know, I've been waiting for any other P0420 diagnostics that I might be able to post since the last one, but there haven't been any except one Dodge Neon that had a rotted vacuum hose on the LDP at the back of the car causing the code. I heard it whistling when the customer pulled up, scanned the car for the MIL code, saw 420, went to the back of the car, saw the hose, replaced in 5 minutes. Problem solved.

Good thing I didn't do that $5 fix, heh? Just pulled some extra vacuum hose out of my tool box.

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