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1997 540i Coolant/Engine/Odd Issues

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I have a 1997 540i that I have owned for about 6 months. It has 104K miles. In the past few days, I've had a number of issues that are probably unconnected, but I am hoping for some help since I can't afford to just start replacing everything.

Known issues at purchase:
-Check engine light for cat converters
-Temperature gauge never moved from cold
-A/C not very cold and cabin air/heat when set to auto never shuts off, lowers or raises fan speed

Since purchase, I use only premium gas (previous owner used regular for years) and I occasionally add an engine cleaner.
-New radiator
-New CCV
-Spark plugs
-Air filter
-New alternator

In the past month or two, now the check engine light will occasionally go off on its own and stay off for about a week. When the light is on, its showing an error for Cat Bank 2 low temperature, sporadic error.

Now the temperature gauge is fluctuating from cold to the middle and back down again in the matter of minutes, usually when stopping and going. Sometimes it will stay on cold and other times it will stay in the middle.

Three days ago, I got the dreaded flashing check engine light and rough idle. Got error code on cylinder 4 and discovered that the ignition coil was bad. Changed it out and no similar issues since.

The following day, the temperature gauge was all crazy and when I turned the ac on, there was no air blowing at all.

I've read a lot about cooling issues and I'm thinking the ignition coil issue was a separate issue, but is there ever a coincidence with BMW? I know I'm all over the place with these issues, but wanted to see if anyone sees a link that I am missing. I'm also new to car maintenance, so I'm not always sure what to look for.

Any thoughts?
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The only possible link I can think of is low electrical system voltage. Check your battery (replace if it's old or in bad shape as a bad battery can ruin an alternator quickly) and alternator voltage to make sure they are in good shape (a new alternator can fail). Make sure the battery terminal connections are clean and tight.

Verify that your coolant level is correct and bleed the system. Replace your thermostat if you still don't get the engine to heat up to the correct temperature. If that still doesn't work then the temperature sensor and/or wiring may be suspect. Verify the age/condition of the water pump and replace it if necessary (it's cheap insurance to do it early).

Unlock your instrument cluster so you can use it to monitor engine temperature and system voltage.

Replace your FSU. If the A/C still doesn't get cold put a manifold gauge set on your AC system to verify your pressures are correct.

The check engine light either means an O2 sensor problem (fairly cheap/easy to replace) or cat problem (as you currently suspect).
I'm also new to car maintenance, so I'm not always sure what to look for.

Take it to a Euro repair shop for diagnostics. These cars are not easy for a neophyte to diagnose and repair.

If you insist on diagnosing yourself, do a search on each individual symptom. These symptoms have been covered extensively.
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