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Old 03-26-2008, 09:44 PM   #1
stanton
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Join Date: Jul 2002
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'03 WRX Wagon
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Default CEL, but no code?

This started happening a few days ago. I posted it in the CEL sticky, but no response:

Replaced spark plugs over the weekend.
30 minute ride to work.
From work I got gas.
5 minutes later, the CEL comes on.
I pull over and check the gas cap. Tight.
Go home, check to see what code got registered. NOTHING!?
Check it again. Start engine, check it again. Nothing.
Clear the CEL. Start engine. Within 15 seconds, CEL relights. No codes!

What the heck could this be? CEL with no code!? It's been 3 days now, with no change. Do you have a clue what might be happening and how I can get over it?

I didn't disconnect the battery when I did the spark plugs. I'm inclined to "reset the ECU" by disconnecting the battery, but I'd rather not right now.
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Old 03-26-2008, 10:13 PM   #2
Mulder
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What did you use to read the code? If you used an AP that may be the reason, it's not a true OBDII reader and won't report all codes properly. Get it scanned with a proper OBDII scanner and the code should show up. You can get a free scan at any Autozone.
Be sure to get the exact Pxxxx code for the most accurate troubleshooting. Once you have the code you can search for posts and other information related to that code.
Do NOT disconnect the battery before you get the code read, or it will be erased and you won't know what it was unless it comes back.
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Old 03-27-2008, 07:28 AM   #3
stanton
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Yeah, that's a good point. I used an Equus 3100. It's a full-on OBDII scanner. Not CAN, but that doesn't matter here. I've used it to scan a bunch of cars (including this one) and get proper P-codes and check "readiness" with no problem. I've seen codes without a CEL, but never a CEL without a code.

The reason I was considering disconnecting the battery is that I'm thinking the ECU is not acting right - only because of this CEL issue.

Last edited by stanton; 03-27-2008 at 07:34 AM.
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Old 03-28-2008, 07:32 AM   #4
stanton
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After reading a few posts about CEL with flashing cruise control light, it made me think - I haven't seen the cruise control light in a while. I checked yesterday, and my cruise control light is burned out. So, I don't know if this CEL is with or without the cruise control light flashing. I'll try to get it fixed this weekend.

Either way, still no code. Anyone?
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Old 03-28-2008, 07:49 AM   #5
Mulder
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The flashing cruise w/CEL only applies to later models with drive-by-wire, it does NOT apply to your 03.
I wouldn't pull the battery until you have exhausted all possible options for getting the code read. Try Autozone, and/or see if you can find someone with a different type of scanner. If you're willing to pay a diagnostic fee, perhaps a dealer can read it with their Subaru Select Monitor.
If there really is no code, an ECU problem is a possibility.
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Old 03-28-2008, 03:18 PM   #6
Jon [in CT]
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I would check to see whether any freeze frame data is stored.
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Old 03-28-2008, 08:46 PM   #7
stanton
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Thanks again for the responses. Good to know that I don't need to worry about the cruise control light, though I'll try to fix it this weekend.

I drove by Autozone to see if they could read something, and viola! P0031. Front O2 heater circuit low. I'll look into it tomorrow, but at least there's a logical reason the CEL came on!

Now, I'm quite upset that my Equus Innova 3100 still does NOT see this code. Sure, P0031 is listed in the OBDII code book included with the unit, but I only wonder why. I believe the unit the guy at Autozone used was an Actron CP9175. Both units are the same $90 at Amazon. I know which unit I would choose if I were in the market. Maybe I am in the market

edit: looking more closely, the Innova 3100 they're selling now has more features that the one I bought several years ago. It's now got more display, CAN, emissions readiness lights, USB, etc. Maybe it is better now. I dunno. So, why didn't they change the model number?

Last edited by stanton; 03-28-2008 at 08:59 PM.
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Old 03-30-2008, 09:34 PM   #8
stanton
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I looked into the situation today. The front O2 sensor wire seems fine from the outside. I double-checked the connector. Still get the code.

At least ecuEdit gets the code, so I do have a back up for the Innova scan tool. I did a log with ecuEdit and found something strange. The A/F ratio from the front O2 sensor - this one being the wide-band - stayed at a rock-solid 14.7:1 (lambda=1) no matter what I did. Thinking about it now, I should have measured the voltage at the connector while the engine was running to determine if the problem is the sensor or ECU. But either way, this tells me it's not just the heater that's messed up.

By the way, this is the most expensive O2 sensor I've ever seen. Autozone wants $220! Advance: $187. A lot more than a generic Bosch that does the same thing.
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Old 03-30-2008, 10:08 PM   #9
Mulder
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You can't check voltage at the sensor, it doesn't work the same as a conventional 02 sensor. Also the A/F your scanner is reporting may not be accurate- I don't think any standard OBDII devices support the data from the front sensor. You need a device that uses the Subaru Monitor protocol to see it, such as a Subaru Select Monitor tool or a Delta Dash.
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Old 03-31-2008, 07:15 AM   #10
stanton
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Thanks for the quick reply. I was using ecuEdit, on a laptop connected to the OBDII port with the Tactrix cable. It reads and logs lots of good information from the ECU - and can read OBDII.

I hear what you're saying about this sensor not being "normal". When I was tuning 1.5 years ago I was comparing the front O2 sensor to an Innovate LC1 wideband sensor which uses a Bosch wideband O2 sensor. The stock sensor is considered quasi-wideband (which is why I'm shocked at the price), and the stock sensor was fairly accurate when comparing the two. The number would be updating constantly, and get richer and leaner where appropriate. Not yesterday though. It never budged from 1.0. I'll try again tonight.
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Old 03-31-2008, 01:43 PM   #11
Jon [in CT]
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Quote:
Originally Posted by Mulder View Post
Also the A/F your scanner is reporting may not be accurate- I don't think any standard OBDII devices support the data from the front sensor.
Any up-to-date scan tool can read PID $24 (Sensor 1 Lambda), which is supported by all US WRX and STI models. You just need to multiply that Lambda value by 14.7 to get AFR.
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Old 04-02-2008, 07:33 AM   #12
stanton
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I finally ran a log to show you what I'm talking about. The top lines are the front O2 sensor. I used ecuEdit. Everything was read through the ECU (OBDII port) except the LC1, which was read through the serial port.



The way I see it, with the O2 sensor always showing 14.7 (lambda = 1.0), the engine would be blind to air/fuel ratio in closed-loop, and would eventually "poison" the learning that goes into open-loop.

I have a new O2 sensor arriving today, but I'm still curious if it's the sensor or ECU. I was avoiding resetting the ECU, because I wanted to extract some timing learning, but due to the bad O2 reading, I believe the information I was holding out for is now tainted too.
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