G35 Overheating Mystery: Thermostat Trouble or Something Else?

Hey there! I had a similar issue with my 2004 Infiniti M35 (I've got some experience working on cars). Initially thought it was the thermostat too, but when I took it to my regular workshop, they found damage to the wiring harness that was affecting the cooling system sensors. The repair cost me around 170€. The symptoms were exactly like yours engine temp rising and that annoying CEL. The wiring was causing incorrect readings, making the system go haywire. What's your coolant level like? And does the temp gauge show consistent readings, or does it fluctuate? These details would help me share more relevant info from my experience. Btw, have you noticed any other electrical issues? That might help pinpoint if it's wiring-related like mine was.

Thanks for sharing your experience! My coolant level seems fine I checked it right after the CEL came on. The temp gauge has been pretty erratic lately, jumping between normal and high. My mileage is at 27804 KM and I just had it serviced last month, so that's why this issue is really bugging me. I haven't noticed any other electrical problems, which is why I was leaning towards the thermostat theory. But what you mentioned about the wiring harness is interesting maybe I should have them check that too. Did your car also make any unusual noises when this was happening? Mine's been pretty quiet despite the temperature issues.

Hey again! Yeah, after reading about your erratic temp gauge behavior, it's sounding even more similar to what I dealt with. Mine was doing that exact same thing jumping between normal and high. Actually, when my car had the wiring issue, it was dead quiet too no weird noises at all, which made it even more confusing! The low mileage and recent service make this really interesting. Mine started acting up right after a service too, which was super frustrating. The mechanic told me that sometimes when they're doing routine maintenance, they might accidentally nick or disturb old wiring these cars are getting up there in age, and the insulation on the wires isn't what it used to be. Quick question when you're idling at a stoplight, does the temperature tend to climb more? That was another telltale sign with mine. If you're experiencing that, I'd definitely push for them to check the wiring harness first before replacing the thermostat. Could save you from replacing parts you don't need to. Just trying to save you from the wild goose chase I initially went on! Let me know what you find out curious to see if our cars are having the same drama.

Thanks so much for the follow-up! And yes, now that you mention it, the temperature does climb more when I'm idling! That's another detail I probably should've mentioned earlier. You've really opened my eyes to the wiring possibility definitely don't want to waste money replacing the thermostat if that's not the real issue. I'm going to take it to my mechanic tomorrow and specifically ask them to check the wiring harness first, especially around the areas they worked on during the last service. You might have just saved me a bunch of time and money here! I'll update this thread once I find out what's actually causing the problem. Really appreciate you sharing your experience it's super helpful when trying to diagnose these older Infinitis. Thanks again!