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Old 12-05-2007, 02:03 PM   #1
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Default DTC PO128

"Service Engine Soon" light came on, so I took it to AutoZone and they pulled the code PO128. They said it could either be a bad or sticking thermostat, or low coolant. Sure enough, the coolant in the reserve was nearly empty...so I bought some Prestone DexCool equivalent (I know I should swap out the Dex).

On my way home to put the coolant in, the light went out. I believe it takes 3 successful driving cycles without this code for the SES light to clear, so I'm hoping it was just a sticking thermostat and it unstuck. Everything seems to be fine now.
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Old 12-05-2007, 02:04 PM   #2
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Disappearing coolant? Early warning sign: http://www.bonnevilleclub.com/forum/...e=article&k=38

Low coolant in the reservoir only will not throw a P0128, it is probably a bad thermostat. Do you have the stock 195 in? Did you check the level in the radiator? What does your oil look like?
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Old 12-05-2007, 11:29 PM   #3
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Nah. Mine used to throw this code all the time in the winter particularly on short driving cycles when the engine failed to reach operating temperature of at least 170 F.
You could have a sticking thermostat or just not driving long enough. It will trip the light after two faults and clear it after two.
I had mine reprogrammed for my 160 stat so now no more SES lights. :P
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Old 12-06-2007, 01:31 AM   #4
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Well, it seems to have remedied itself so I'm hoping that its something like you're describing SilverBullet, especially since this didn't crop up until the weather was real cold. However, I think I will check the Upper Intake stuff just to be on the safe side, I'd much rather fix it now than be screwed later on because I was careless.

The thermostat is stock as far as I know, I doubt the previous owner changed it. Oil was fine the last change (last month) and I'm not sure about the level in the radiator itself.

Thanks for the link Haro.
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Old 12-06-2007, 03:21 PM   #5
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I get the same code. Its a sticking thermostat.

Mine sticks open at times so I'm not too worried. The 180 goes in when a PCM goes in.
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Old 12-06-2007, 03:34 PM   #6
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Ironic thread!

I scanned my buddy'* 04 Grand Prix last week and found code 128. This past Sunday I swapped in a new thermostat for him. No more check engine light!
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Old 12-06-2007, 05:11 PM   #7
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Tis' a bit ironic, haha. Glad the general consensus is a bad/sticking thermostat as opposed to the more serious options.
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