Might wanna grab the service manual. I bought mine for my 2002 on ebay on cdrom. Just search Hummer Repair Manual and you'll find one. It is the only way to go and will save you hours of time and 1000s of dollars. Anyway just my two cent. Good luck with your hum hum.
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Probably a cooling fan issue or maybe a stuck thermostat.. change thermostat and check that cooling fans engage..good luck...also when you fill up the resorvoir let it be for a while and check to see if level is down, sometimes it takes a while for water to seep into radiator, i assume you dont have a radiator cap?
Sounds like you have a faulty head gasket or maybe a cracked head.This can cause overheating and the check engine light to come on.The code for the o2 sensor is probably caused by coolant getting into the exhaust system and coating the sensor.The coolant fan may work but it cannot remove enough heat from the rad to make the engine cool off.Try some engine coolant stop leak to see if it will help.This maybe only a temperary fix.Let me know if you need more.MOE
Check coolant level, and possibly replace thermostat. When a thermostat gets stuck in the closed position, coolant does not circulate thru motor. Gauge stays midrange when driving, because motor is being cooled by air and or the fan. If car seems to operate completely normal, then the gauge is faulty.
If this truck is not overheating, then I would suspect the temperature sensor, if it is overheating then this must be repaired first. The sensor should be mounted in the intake manifold. Hope this helps, let me know
Your thermostat may be stuck in a partially opened position. This would account for the low temp while driving.You can check it by removing it (it should be closed when at room temp, if it's not, it's defective) and dropping it into a pot of boiling water, it should open completely within a few seconds. Also make sure your coolant level is correct, and when idling, check to make sure the engine cooling fan is kicking on when the engine temp rises.
If you disconnect the guage sending unit what happens? Are you sure that there is only one sending unit, many Japanese cars use 2, one for normal zone and one for overheat, the last makes the guage quickly go to hot to alert the driver of a problem before the engine is ruined, let me know what you find.