1. After 10 ~ 15 minutes driving, temperature reading only reach a quarter of full scale. Normally, it should reach half of the full scale. It is too low. What can cause this? I got the coolant flushed and refilled recently.

2. I also got a error code: 043. That is "Spark Control Module" fault. Where is this guy? I may need to replace it.

This car has 105k miles and runs good except these two problems.

Any help will be greatly appreciated

1. After 10 ~ 15 minutes driving, temperature reading only reach a quarter of full scale. Normally, it should reach half of the full scale. It is too low. What can cause this? I got the coolant flushed and refilled recently.

2. I also got a error code: 043. That is "Spark Control Module" fault. Where is this guy? I may need to replace it.

This car has 105k miles and runs good except these two problems.

Any help will be greatly appreciated

Sorry for my typo. it is 95 Lumina sedan instead of 98'

Most likely faulty thermostat. That's an easy fix.

Most likely faulty thermostat. That's an easy fix.

I just joined and I'd like to know how you change this elusive thermostat. I can't find the thing! I have a 98 with the same problem.

Where the upper hose goes into the motor...looks like it won't come out with the pipe in place (3.1 motor) but it will, leave the bolts in the housing and take it off together...

Bleed the system completely when filling or temp will spike when you drive it.

The ones that I have driven temp rarely goes more than 1/3 of gauge range unless working it hard at low speed.

To be fair, almost every 95-99 3100 GM I've worked on has the temp scale that's way off. Typically, they fall between 1/4 and 1/3 of the total gauge reading when the engine is at normal operating temp. This includes the Lumina, Cutlass, Grand Prix, Grand Am, you get it. Unless you've noticed a recent change in the needle's position, like it used to be halfway now it's only 1/4 or similar, then it could be the thermostat. Don't eliminate the sensor wiring harness, either. Though the sensor itself hardly ever fails (in my experience), the wiring gets brittle, cracked, or can wiggle its way loose.

Just throwing my two cents in..

To be fair, almost every 95-99 3100 GM I've worked on has the temp scale that's way off. Typically, they fall between 1/4 and 1/3 of the total gauge reading when the engine is at normal operating temp. This includes the Lumina, Cutlass, Grand Prix, Grand Am, you get it. Unless you've noticed a recent change in the needle's position, like it used to be halfway now it's only 1/4 or similar, then it could be the thermostat. Don't eliminate the sensor wiring harness, either. Though the sensor itself hardly ever fails (in my experience), the wiring gets brittle, cracked, or can wiggle its way loose.

Just throwing my two cents in..

usually if your thermostat is stuck open your toes will freeze....

Which engine?

3.1 ?
3.8 ?

On my 3.8 I replaced the t'stat 3 times and still had low temp reading. I then replaced the sending unit under the t'stat.
Reads as it should now.

If I remember right, the t'stat is in the housing for the return hose attachment to the radiator intake.

The spark control module is underneath your set of three coil packs. On the back side of your motor are where your three coils are. The "rail" if you will, that they bolt on to, would be your ICM or Spark Control Module.