Bad gas milage and check engine light.

[Diabolic1]

I noticed that this last tank of gas I only got about 22 mpg in my 03 SI. The check engine light came on about 1/2 way through the tank. Scanner said "catalyst"??? Then after 2 run cycles the light went off and has not come back on sense. Bad gas? Converter doesn't smell clogged or like sulpher. Guy with the scanner said it could be o2 sensor. Is a failed converter a common place with these cars at 90K? Where is the best place to get one? I found just the converter for an 03 Si on Parts Train for a little over $200. Is there another option? Thanks!

[dann_04]

get the actaul number from the cel and post it up, autozones references can be off as hondas cels are a little different than the generics they have. Both an o2s and a clogged cat would cause bad mileage, but i would narrow it down and find out which one it is before i went and started replacing parts. You only have 90k and it's an 03, do you still have a warranty? Just asking because i bought my 03 used and have a 100k warranty. Also i would imagine it's the o2 sensor, they tend to go bad before the cats do. Have you lost alot of power? A bad cat can feel like a clogged fuel filter where the car loses alot of power and bogs down. An o2 sensor normally just throws a cel and your mpg's go to hell.

[Diabolic1]

I don't know what it's supposed to feel like. This is my 1st non V8 car. CEL is off but I just filled up and got 19 mpg all city and driving like a grandma. Should I find someone with a better scanner? Would Honda dealership be my best bet? Would they be able to tell me exactly wether it's the cat or the o2 sensor. The car has only 1 cat and 2 o2 sensors? How much is a high flow aftermarket midpipe with cat? Does it keep the CEL light off. This car is stock and my daily driver. I drive a Ford Lightning as my toy.

[jtyler05si]

you are from texas... do they do inspections? they dont here in OK....
im getting rid of my cat and putting an o2 defouler in there...personally i think it is a waste of money to buy replacement cat...

[Diabolic1]

I just pulled the code although the light has never come back on. The code is PO420. Does this sound like the second o2 sensor or the cat? Would the delaership be able to tell me exactly? I have to pass emissions here in Dec.

[robhep3]

I just pulled the code although the light has never come back on. The code is PO420. Does this sound like the second o2 sensor or the cat? Would the delaership be able to tell me exactly? I have to pass emissions here in Dec.

p0420 is a catalyst efficiency code. without actually getting in there and testing things, there's no way to tell whether it's a catalytic convertor or a secondary O2 sensor....

Unfortunately, IIRC the secondary O2 sensor isn't going to cause poor running or low MPG... The primary O2 sensor would have to be malfunctioning as well to cause the types of problems you're describing... You can always try installing a mechanical O2 sim in the downstream O2 sensor bung... There used to be a DIY on them somewhere. I'll see if I can find it anywhere...

EDIT: Link to the link (http://www.ephatch.com/forum/showthread.php?t=44645)

[Diabolic1]

My check engine light has stayed off. I ran out half a tank of gas yesterday and got 21 mpg in town only shifting between 3000 and 3500 rpm. The car has 90K miles. Should I go ahead and change the plugs already? Anything else I'm missing? Will look at the air filter too.

[Rep]

My check engine light has stayed off. I ran out half a tank of gas yesterday and got 21 mpg in town only shifting between 3000 and 3500 rpm. The car has 90K miles. Should I go ahead and change the plugs already? Anything else I'm missing? Will look at the air filter too.
Maybe I missed it. Are you still using the same bad catalyst? 21 MPG is crap, there's something very wrong with your car.

you are from texas... do they do inspections? they dont here in OK...
Depends on the county.

[Diabolic1]

Maybe I missed it. Are you still using the same bad catalyst? 21 MPG is crap, there's something very wrong with your car.

Depends on the county.

Yes, that's what I'm saying. I'm getting 20ish in town and almost or right at 28mpg on the hwy. I don't want to replace the cat if it isn't defective. I don't smell any sulpher from the converter like it's clogged. The CEL has gone off and stayed off.
The rear o2 sensor would not effect my gas milage, so that can't be it. The front o2 could cause it if it failed and started making the car run really rich and dumping excess fuel that got into the cat.That would reduce my mileage, but why not on the hwy too? A cylinder misfire could throw the code too. I don't want to replace a $1200 converter if it's just a $200 o2 sensor, but how do you tell. I don't like throwing parts at a car until the problem goes away. I want to diagnose the problem and replace the actual culprit. I will replace the plugs regardless because of the mileage.

[Rep]

I know what you mean about throwing money at the car and hoping you catch the problem. A used cat for these cars is probably like $40. Although it is illegal to sell a used cat commerically. Might have to find a local. I think the base RSX cat is the same as well.

[spike 03EP3]

My car did the same thing, it was caused by fuel sloshing around in the tank when I was Autocrossing. I basically got air in the line and it tripped the CEL, all I did was disconnect the battery and filled the tank and all was well. Maybe all you need to do is reset the computer.

EDIT: didn't see your post about the code you found, first thing I'd try is reset the computer, plus it doesn't cost anything to do. Depending on the code thrown, the computer will sometimes go into a "safe mode" to keep it from causing more damage. I couldn't get my car passed 4,000 rpm and then the limiter would shut it down.

[Diabolic1]

I pulled the 15 amp fuse for the ecm from the box under the hood for 15 minutes. Would that be long enough? Going to try a different driving style in town this week and see what the results are. Also doing an oil change and plug change this weekend. I think that dispite what some say on this board, realistic mpg for a stock Si and the average driver is 23-26 in town and 28-31 hwy. Maybe a fluke 32+mpg every once in a while, but overall 30mpg or less.

[spike 03EP3]

15 minutes should be enough, but I like to disconnect the battery just to make sure. Your numbers for average mileage looks right, I get about 25 to 26 just around town and 30-32 on the highway.

[dann_04]

If you're at 90k i would check the valves and replace the plugs. properly adjusted valves are a wonderful thing.

[Diabolic1]

Would that the valves need adjusting? I thought they were hyd tensioners or something. I haven't adjusted valves since I sold my 70 Chevelle with mechanical lifters.

[spike 03EP3]

A valve adjustment won't make any difference with the problems you're having. They do need to be adjusted every now and then though, you'll know when you'll need one because you'll hear them ticking and clacking. Had mine done at 40k miles, much quieter now.

[Diabolic1]

I do hear some valvetrain noise right at startup, but it goes away quickly.

[dann_04]

Honda says adjust the valves at 100k, but mine were way of at 60k, one was super tight, and one would fit a .014 when it should have been .008. Mpg's were better after i adjusted them.

[Gargola]

My car gave me a CIL this weekend ( Exactly one week after I bought it), with the same codes you are experiencing.

p0420

On my way to my appointment at Midas, the CIL turned off.



Midas checked it out, along with a slew of other things I got going on with the car, and they said both my 02 sensor banks were pulling the code.

They recommended I replace the first o2 sensor first, if I didn't want to replace both at once or fork up more $$$ so they can troubleshoot further.

I am getting the same gas mileage you are, but I don't have 3rd gear, so technically I would get better gas mileage.

I have many issues to deal with on my EP3, and I don't plan of looking into the O2 sensor issue any further until I address other more important issues (Such as the third gear issue).

Not sure if the information given to me by Midas is any good, but I thought I would share it all the same.

Roberto