So I’m looking to figure out what may be the cause of this issue:

my 08 Chevy hhr recently started having an issue. The coolant Temp Gauge randomly started flatlining meaning it reads all the way cold. It would go back to normal (midway where it’s supposed to be) and then it’d continue to flatline back and fourth. It never overheats. Just goes to reading cold. I checked coolant level which was fine. I brought it to advanced auto to try and see if any codes popped up for a bad sensor or anything along those lines and nothing. The Heat and ac still work fine. However when the gauge flatlines you can feel a change in the car it almost surges or feels like it’s having a harder time switching gears. Say I’m going 65 on the highway and the gauge flatlines - the car will start almost revving higher. But I’ve driven for a couple of days with this issue. Sometimes the gauge will be flatlined for very long periods of times. I have no idea what I should do to address the issue. Does it sound like it could be electrical? Any help would be appreciated.

 

to the forums!

98% you need a new thermostat, ACDelco part number 131-158, accept no substitute.

2% you need a new engine coolant temperature sensor.

 

X2 ^^^^^^, welcome to the site! And just in case here’s a how to thread

https://www.chevyhhr.net/forums/how-...l-p0128-24493/

 

My alternate guess is either a loose wire to the temp sensor or a bad temp sensor. The "bad t-stat" usually appears on the first warm up and sometimes goes away after a second warm up, like comes on on the way to 7-11 but when get back in with your six pack it is gone.
The way P0128 works is by timing how long it takes to get to 180F, that makes a flatline gauge. If the t-stat is stuck open on a 72F day it takes too long, it is already part of the way there after you stop for a few minutes.
A loose connection or a bad sensor will flatline after a complete normal warm up.

 

Yeah so the heat and ac both work still. That’s why I’ve had people trying to tell me it’s definitely not the thermostat. They are saying that if it was the stat then the car would’ve overheated by now.

however I’ve noticed the car starts to run a little rough when the gauge is flatlined. Im not sure if there’s a possibility that the car is misfiring and needs new plugs and coil packs? Could that potentially cause the temp gauge. Sensor to flatline and go back and fourth? I need to bring it to my mechanic asap it’s definitely been nothing but a headache I’ll say that much.

another issue I’ve noticed is after a while of driving with the gauge flatlined…when I stop at a light and then accelerate again the car will jerk as it shifts. I’m not sure if that’s a misfire issue or what. These issues only started after the temp gauge started acting up.

 

That’s the computer getting your attention, there is a problem. If it’s code P0128, it is the thermostat. But what do we know?

 

AS explained in #5, there are 2 possible reasons for the gauge to flatline. Both should cause a CEL with code P0128 and AC compressor not engaging and the computer running in open loop mode.
Have you looked at the wires or the t-stat or the coolant level under the pressure cap? I hope you are looking at the digital temperature? You have not mentioned the CheckEngineLight. How do you know it is not over heating if the gauge isn't working? Does the temp ever get above 180F?

Surging etc is an indication of a code, maybe a pending code that has not set the CEL and only shows up when the engine is acting funny.
I will bet $0.02 that if you look at the t-stat you will find that the inner gasket is dislodged and stuck in the valve, holding it open.

 

In which case it’s not gonna overheat cause the thermostat is stuck open.

 

Wanna 2nd opinion? Your thermostat was sticking open intermittently and now it's worse. That condition won't stop you heater or a/c from working, although if it were still February you might not be satisfied with the heat output.

Even if that's wrong, replacing the thermostat is just about the cheapest easiest way to attack the problem. Wouldn't hurt to look at the wires and plug for the temp sensor, take it out and clean it or replace it, if you don't mind the extra work.

 

sorry for the hijack, but my gauge and digital readout worked fine this morning on a 270 mile trip, then about 4 hrs later, the gauge stayed at zero, and got the 3 lines on the digital readout. Stopped for about 1/2 hour (menards) and then stopped for fuel, same thing. When I got home, (no CEL) I plugged in my scanner and it said there was a 'pending P0128 code low thermostat temp reading (I think) I did a reset and the gauge jumped up to normal reading. The trip this morning, temp was right around 176-180 the whole way there and back. On the afternoon trip, the A/C never got cold, just warm air like outside. I tried the wire jiggle and everything seems tight, but the gauge did not work until I used the OBDII scanner to reset. Did not get the CEL like I did last time.


Just a side note, There seems to be an extra hump in the floor behind the drivers seat. Me thinks the jack wasn't on the frame when I changed the brakes... OOps!!