Egr low circut malfuntion

If the coolant level is low in the engine it can cause heat fluctuations from the heater, as well as an incorrect reading on the gauge. Make sure the coolant level is correct in the engine.

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These details will help.
Thanks.

your coolant temp sender could be bad. locate the temp sender. this is a 1 wire sensor. the other sensor in the block has 2 wires and starts the cooling fan.

You might check the coolant level after the engine cools,if it's low add 50/50 mixture of coolant and water. Keep an eye on the level to see if it happens again. You might have a small leak in the system. Good luck

Definitely check your fluid level in your radiator. Top it off all the way. I've seen that happen multiple times on cars that were low on anti-freeze. If that doesn't work try replacing your thermostat. It's cheap and the easiest place to start if topping off the fluid doesn't get you fixed up.

Your best bet take it to a qualified repair shop that can do the diagnostics for the codes, no code tells you to replace a sensor . Viewing scan data on a professional scan tool an doing diagnostics
CatalystSystem Efficiency Below Threshold(bank2) ! This means you need to have a catalytic converter replaced .
080 EGR and Catalytic Converter Tests EGR Test

Check for low coolant, with an intake gasket leaking. Most of these I see have an intake gasket leaking coolant which when the coolant gets low, will run hotter and fluctuate the temp gauge.

check coolant level ,must be low ,chck for coolant loss

The shut off valve on the heater core was left shut during the repair on your coolant system. the repairman forgot to re open the shut off valve . this is the problem. open the shut off valve and, the air will become hot again. as for the gauge fluctuations, the actual gauge may be malfunctioning.

check your coolant level on the radiator