Running to cool...check engine light
Thread Starter
Burning Brakes
Joined: Jun 2013
Posts: 1,186
Likes: 16
From: Orlando Fl
Running to cool...check engine light
So I have had the car about 2 weeks and this is the first time I have driven in cool temps...It was 64 last night. City driving is fine 190 coolent 195-200 oil. However on the highway the coolent dropped to 152 and 168-172 on the oil. I know it has a 160 thermostat due to the modification. Cam, twin turbos and I do live in central Florida so these temps are not normal but for a few months and mostly at night. Should this be a concern?
Le Mans Master
Joined: Feb 2012
Posts: 6,644
Likes: 479
From: OK
Le Mans Master
Joined: Jun 2007
Posts: 7,430
Likes: 4,829
From: BEST Corn-Nation
2015 C5 of the Year Winner
I have a Grand Prix I use for work and I have a 180 degree t-stat in it. It runs cooler during the winter months and I get that light once in a while. I usually just clear it with a hand held scanner that I leave in the car. Been doing this for a few years now and the car still runs great. No issues that I've noticed and it's fast approaching the 200K on the mileage.
Burning Brakes
Joined: Dec 2011
Posts: 858
Likes: 75
From: Medford, Oregon
I have the 180 degree stat in my 99' and does run low temps when cruising on freeways on cool mornings (60-70 degrees) and temp reading have gone down to showing 156 at times, but never has thrown a code or have check engine light come on, Hmmmm !! Waiting to see more responses to this issue. 
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Le Mans Master
Joined: Mar 2004
Posts: 8,700
Likes: 298
From: Ft. Lauderdale Fl
Pull the codes to determine the cause of the Check Engine Light.:
OR
http://www.c5forum.com/ayc/dtcdic.php
Post ALL your codes including the H or C or HC that follow each 5 character code.....
Running the engine that cold for long periods of time is NOT good as the condensation that normally forms in the engine does not have a chance to boil off (needs to reach at least 212 oil temp for that to happen. The cold engine temp MAY be related to the CEL but only the codes can tell us for certain..... Dozens of different codes that can set off the CEL so you need to know what codes are stored.
Many that run 160 thermostats in hot weather will switch to a standard stat for cold air temp conditions such as later fall through mid spring if they drive the car in those temps a lot.
OR
http://www.c5forum.com/ayc/dtcdic.php
Post ALL your codes including the H or C or HC that follow each 5 character code.....
Running the engine that cold for long periods of time is NOT good as the condensation that normally forms in the engine does not have a chance to boil off (needs to reach at least 212 oil temp for that to happen. The cold engine temp MAY be related to the CEL but only the codes can tell us for certain..... Dozens of different codes that can set off the CEL so you need to know what codes are stored.
Many that run 160 thermostats in hot weather will switch to a standard stat for cold air temp conditions such as later fall through mid spring if they drive the car in those temps a lot.
I run a 160 tstat and change it out for the stock twice a year.
Advanced
Joined: Nov 2016
Posts: 71
Likes: 6
It has been cooler weather lately from the 30s to low 50s and my 03 c5z will randomly throw the p0128 code. The car mainly stays in the 160-165 range when driving. It has cam, lts, intake etc and 160 thermostat. I read this code is common so is my options just to keep clearing the code or switch back to stock thermostat? Will letting the car warm up idling before driving help at all?