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PO430 disappeared after compression test??
Topic Started: Dec 4 2016, 05:04 PM (259 Views)
elgecko
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Hey all. Earlier this week my Geo wasn't starting in the morning or at night. It would start in the afternoon so I assumed it had to do with the temperature. At the same time, it started throwing a PO430 code at me. I got some time yesterday to do a compression test and here are the numbers:

Dry: 190, 180, 110
Wet: 190, 180, 120

Obviously some work needs to be done (a full rebuild?? :'( ) but what was unexpected is that after the test the code went away. Keep in mind, I accidentally put in more than a teaspoon of oil in the first cylinder so it was smoking like crazy for a while. Why would the code disappear if I was burning oil so crazily? Why would the compression test eliminate the code at all? I went ahead and did an oil change as it was due, but other than that no work has been done.

Second question, do I need a full rebuild? I went through this whole deliberation last year at almost this exact same time and BBowens knew a guy out here that could do it but I deployed before anything took place. My compression numbers last year were:

Dry: 210, 205, 130
Wet: 180, 180, 120

So things aren't exactly getting better...
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Silver2K


P0430 is inefficient cat. The computer monitors the second O2 sensor to check for oxygen, which is a sign the cat is working. An inefficient cat will not keep the car from starting, or cause any other problem. I suppose there are some things that might fool the computer into thinking there is a bad cat which might cause drivability problems, but I can't think of any.

If you Google fixing a bad cat, you will get suggestions like taking the car on the freeway to heat it up which will help burn off crude.

Your low compression on one cylinder is likely a bad exhaust valve. The bad valve might be degrading (overheating) your cat.
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elgecko
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Why would the code go away if I'm burning more oil than normal (or was, at the time)? Wouldn't that make the code come on instead?
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freegeo
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What year car us this on? 91 can't have a PO430.
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myredvert
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myredvert

While it's likely the DTC is related to your low compression, it's also unlikely the code clearing was related to the compression test, unless it cleared after 3 passing cycles that occurred when all the enable criteria existed for running the dtc (also assuming you are talking about a mystery year OBD II Metro not listed in your signature). If it cleared the first time you drove it after the compression test, then it had already passed before that and was waiting for the required number of passing cycles to occur before clearing the dtc.

Couldn't tell you what happened unless (as nothing more than a wild ass guess) that some of the contents may have shifted, altered, burned up and exited the system, etc. in a way that made the system think it was working acceptably well enough.

What Metro year/model has this issue by the way?
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elgecko
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That's really weird, because now I see what you're saying, the P0430 is OBDII. I swear it was blinking 4 times and then 3, so now I am even more confused.

Edit: this occurred on my '91.
Edited by elgecko, Dec 4 2016, 07:33 PM.
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elgecko
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Myredvert- that makes sense. How is a cycle determined?
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myredvert
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myredvert

[edit] just saw your post about the year... let me take another look at that...

In the mean time, you may want to take another shot at your flash diagnostics - as far as I know the 1991 doesn't have a DTC 43 either. :hmm


To answer your question about cycles in general, the criteria for cycles and for the enable criteria necessary to run each DTC can be quite different, and are listed in the Factory Service Manual for each specific DTC . There are "ignition cycles," "warm up cycles," "passing cycles" to name a few off the top of my head. A passing cycle occurs when all the enable criteria are met, the DTC is run, and a fault is not detected.

I don't have access to a 98+ FSM, but for example for the 97 some of the information regarding a P0420 DTC is:

Conditions for Running the DTC
• Engine coolant temperature between 70°C (158°F) and 110°C (230°F).
• Intake air temperature between -10°C (14°F) and 50°C (122°F).
• Barometric pressure greater than 75 kPa.
• Fuel tank level greater than 25%.
~ Engine Speed between:
- MT 2300 and 4500 RPM.
- AT 1500 and 4500 RPM.
• Calculated load value between 26 and 80%.
• In closed loop mode 360 seconds from engine start.

Conditions for Setting the DTC
• Delay time average of H02S 2 response.
• Conditions present for 45 seconds.
Action Taken When the DTC Sets
• The Malfunction Indicator Lamp (MIL) will illuminate after two consecutive ignition cycles in which the diagnostic runs with the fault active.
• The PCM will record operating conditions at the time the diagnostic fails. This information will be stored in the Freeze Frame buffer.

Conditions for Clearing the DTC
• The MIL will turn OFF after three consecutively passing cycles without a fault present.
• A History DTC will clear after 40 consecutive warm-up cycles without a fault.
• DTCs can be cleared by using a scan tool or by disconnecting the PCM battery feed.

If you want to know how the systems function and have access to the most comprehensive diagnostic procedures for our cars, nothing compares to the factory service manual.

It's also possible that the DTC was being set for a very particular combinations of operating conditions, but passing cycles subsequently occurred but the conditions that triggered the fault in the first place didn't exist for the required number of passing cycles. FWIW, I'm not a fan of intentionally giving away possible useful information in order to clear the ECM/PCM memory to clear a code except under unusual circumstances. I would rather make sure I drive it within the enable criteria for running the DTC enough consecutive times to see if the fault has cleared on its own after repairs. at the very least, if the memory has been cleared, make sure you operate the car within the specified parameters to see if the DtC will set again. It's not unusual to do a repair, clear the memory, then think the fault is gone but in reality maybe it's just that the enable criteria hasn't been met to run the DTC sufficient consecutive times to reset the DTC. Then one day it mysteriously returns.... :O In reality the fault may never have been gone, the conditions necessary to clear it just didn't occur, i.e., it wasn't detected becasue the test never ran, or didn't run enough consecutive times to set it...

Another downside of clearing the memory is that the long term fuel trim tables will be cleared, and a re-writing period will be necessary during which some minor driveability issues may occur.

If you don't have an FSM for your particular car, it's invaluable provided you get into it enough to learn to use it properly. In the mean time, if you need any procedures/diagnostics you can ask freegeo to post them for you.
Edited by myredvert, Dec 4 2016, 08:37 PM.
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elgecko
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myredvert- thanks for the great info. I actually do have the FSM, but like you mention I haven't gotten into it enough to find everything right away. I eventually found the table of codes and I see that a code 43 isn't even listed. I feel like an idiot because I swear it was flashing 43 but now I see it isn't even possible. For what it's worth, I'm getting 12 now.

My thought is that it's like you said and I must have cleared the cycles after getting it started again. I just don't know what I cleared...
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myredvert
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myredvert

Quote:
 
For what it's worth, I'm getting 12 now.
:thumb See, all fixed. ;)

Just make sure you clearly identify the long pause (3 seconds) as a starting reference - it's not unusual for a DTC 21 to be read as "1-2." The very short single flash at the instant you begin the flash diagnostics is not part of the number sequence, i.e., it's not a "1."

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elgecko
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myredvert
Dec 4 2016, 11:07 PM
Just make sure you clearly identify the long pause (3 seconds) as a starting reference - it's not unusual for a DTC 21 to be read as "1-2."
Yeah, I think I'll be a lot more careful from now on :shake

Thanks for the info, it's greatly appreciated :thumb
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