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The sensors in my car aren't accurate

I’m slowly fixing up a 1998 Plymouth neon I only bought it because it gets amazing gas and it only had 77k original miles on it. I’ve replaced quite a bit of stuff on it. Finally got it to pass inspection so now i just want to slowly make it completely perfect and not so tempermental.
What I’ve fixed/replaced:
Catalytic converter
O2 sensor
Exhaust pipe
Front brakes
Passenger inner and outer tie rod
Oil change
Spark plugs

I’ve put a lot in this car. And I know some of what I listed above isn’t necessary to my problem I just thought it might help. Anyways I have 3 sensors in my dash gas, speedometer, and temperature. The spedometer will work on and off it’s completely random and I’ve noticed it’s usually off at least when I’m going slow I notice it’s off I’m not sure about higher speeds. My temperature gauge rarely works. I know my car isnt over heating though it just usually stays on C or a little above it it rarely makes its way to being in the middle where it should be. And the latest problem is my gas gauge. On my way to work today I noticed my car went from a half tank to being in between half and a quarter. So I kept an eye on it and it kept going up to like a little above half to in the middle of half and a quarter tank. I really have no ideas on what my car is doing I just really need these fixed. Also sorry i don’t know what this part is called but not the odometer but the other number thing that tells you how many miles you’ve traveled in a trip is off it will say ive traveled 30 miles when ive only gone around 20 i dont know if that plays into the other stuff sorry I don’t know what to call it all the lights work on my dash though like oil light and check engine and turn signals

Sounds like a classic ground fault. The grounding points on your Neon likely have gotten rusty in the nearly 20 years of life. Your mission, is to find them and clean them nice and shiny. The dash grounds are likely under the dash hidden in many hard to find places. The car runs well and the engines computer uses things like coolant temp to run the engine so the sensor is likely not the problem. Sometimes there is a second temp sensor just for the gauge so that could answer one problem but not the other 2.

I’d bet your dash and gauge problems go away if you clean the grounds.

I suspect this was a flood vehicle and the troubles may never end.

Ask your shop to check the cluster harness 10 pin connector. For the 2L vin C engine, Pin 5 ignition voltage, pin 6 battery voltage, pin 8 is chassis ground. This may require the cluster be removed first.

Your odometer problem is likely caused by whatever’s making the speedometer erratic, so focus on the speedo problem first. One of those infrared laser guided thermometers would allow you to verify the coolant temperature is correct, and that it is just a gauge problem. Even just plunging a plain thermometer into the radiator would help to verify that it is just a dash gauge problem, but you’d have to wait until you could safely remove the radiator cap to do that.

One other test you could ask your shop to do, the dash temp gauge should go to its lowest position if you disconnect the coolant temp sensor.

Monkeying around with the electrical system should only be done with the battery negative disconnected first, to minimize causing other, perhaps worse, problems. .

These cars have problems with the instrument cluster circuit boards. The next time the gauges drop to zero slap the dash above the cluster, the gauges may begin working again.

You can replace the instrument cluster with a new or used unit or have someone re-solder the solder joints on the circuit board.

The inaccurate fuel gauge may be due to a failing level unit in the fuel tank.