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Old 02-10-2009, 03:57 PM   #1
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Interpreting data from OBD2 scanner

I just bought a OBD2 scanner (Memoscan U581) for $108 from eBay and was playing with the Live Data feature. I currently have no MILs and my car has no driveability problems (but my catalytic convertor glows a little red every time i drive it, which is part of the reason why I bought this)

I have a '05 Mazda RX-8 6MT with 57,000 miles, and Racing Beat intake, cat-back exhaust, and street ECU flash. Using about 70/30 coolant w/ water wetter.

Here were my live data readings/observations on a ~25 mile drive to work:

Drive cycle:
- starts in open loop (on the scanner: "OL, drive conditions") until car is warmed up, then alternates between "OL, driCL using HO2S" and "CL, using HO2S" during city driving and freeway cruise. I'm not sure what the phrases "drive conditions" and "OL, driCL using HO2S" mean though.

Coolant Temp:
- at car start up, temp was 24 C (75 F), which rose quickly and stabilized at 82 C (180 F) (doing freeway speeds) in about 7 minutes.
- when the car's coolant temp gauge reached the "middle" where it stops, the scanner's coolant reading was about ~70 C (158 F). So when the gauge hits the middle it takes a minute or 2 to reach stable operating temperature.
- when stopped at traffic lights during city driving, the temp crept up to 88 C (190 F), but would quickly go back to 82C when moving again.

STFT - Short term fuel trim
- seems normal when driving around the city. Oscillates between -5 to 5% and is near 0% when stopped.
- a little different while on the freeway, not sure if this is a problem. The value is stable between -5 to 5% while cruising, but if I let off the gas then tip-in on the gas, even just a little, the value immediately shoots up to 20-25% then quickly comes back down. I assume this is a lean spike reading due to the lag of fuel being injected? Driveability is good so I assume nothing is wrong.

LTFT - Long term fuel trim
- stays at 1.5% in the city, and goes to 3.8% on the freeway.

Throttle position:
- foot off the pedal: 16.6%, foot to the floor: 78.8%
- slowly pushing the gas pedal down the value rises slowly from 16.6% to about 24% for the first half of travel, then it quickly shoots up to 78.8% for the last half.

Mass Airflow in gm/s:
- reading hovers around ~5 gm/s during cruising, and goes up to some higher number smoothly depending how much I gas it.

HO2S12 (v): I'm guessing this is the voltage reading from the heated O2 sensor in the exhaust manifold. This was the only O2 sensor reading. None listed for the O2 sensor in the cat I guess?
- I think its fine, voltage is switching quickly between ~.15 and ~.85 when I stab the throttle and it oscillates about .5-.6 when cruising.

HO2S12 (%): I don't know what this is for?
- stayed at .7%

Secondary Air:
- reading was "Error", but I'm not sure if it just failed reading it or if something was wrong. The I/M Status screen states that the Secondary Air System is "ok" though.

Hopefully these are good readings for my car. Please let me know if there is anything wrong or if there's other behavior I should look for.
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Last edited by lshu; 02-10-2009 at 04:08 PM.
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Old 02-10-2009, 04:08 PM   #2
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If your cat is glowing red under normal driving conditions, it might be clogged. I'm not sure what sort of OBD2-identifiable error codes that would give you.
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Old 02-10-2009, 04:13 PM   #3
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It is a brand new OEM cat from Mazda, installed in Dec. '08 from car repairs (I killed my Mazsport midpipe by hitting a metal grate on a driveway). The cat has being doing the glowing thing since that time, but I haven't really felt a loss of power yet. So I don't think the cat is clogged. I think the cat will die if this keeps going on, so I'm just gonna get Racing Beat's resonated midpipe when it comes out.
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Old 04-19-2009, 07:40 PM   #4
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good read. will be informative when i get my scanner. what i found interesting is

Throttle position:
- foot off the pedal: 16.6%, foot to the floor: 78.8%
- slowly pushing the gas pedal down the value rises slowly from 16.6% to about 24% for the first half of travel, then it quickly shoots up to 78.8% for the last half. .


wondering why it did not read 100$
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Old 06-25-2009, 06:35 PM   #5
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I have a engine light on, autozone said it was a failed HO2S 12 sensor. I dont really know anything about that. Could someone explain and help with the problem.
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Old 06-26-2009, 08:18 AM   #6
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Old 06-26-2009, 08:18 AM
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data, ecu, fuel, interpreting, long, ltft, memoscan, o2, oldrive, position, rises, sensor, term, throttle, trim, u581

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