> curious if the spare is salvageable

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it depends.

small leak in the threads? plug it

small leak on the sidewalls? game over man. game over.

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also pictures are needed before assessment.

https://hyundaithetaenginesettlement.com/faq.html

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there seems to be a major class action lawsuit for your model of car.

theoretically you can use any kind of thin oil lubricating fluid for anything.

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realistically speaking, no absolutely not. do not do this.

you do not to be messing with one of the most complex and expensive to fix parts of the car.

leave it alone and use the proper fluid with proper additives.

yeah sure. try to keep it the same type of coolant. make sure it meets or exceeds vehicle standards. follow manufacturer dilution procedures.

that’s an engine issue. a serious one. there’s discussions on hyundai forums about how the sonata needs an Engine Control Module update from the dealership and there was a recall going on.

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this is way above DIY stuff and some garages may struggle with it.

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https://www.hyundai-forums.com/threads/p1326-error-limp-mode.656891/

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https://hyundaithetaenginesettlement.com/faq.html

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this is dealership/hyundai specialist garage level stuff imo.

get us the code from the OBD2 and then we can help you.

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this is strange because its a 7 year old car. its not that old.

if you replace spark plugs and its still causing issues, it would likely be a bad ignition coil.

Yup sounds like the master cylinder and the braking system itself has gone bad if there’s no leaks or loss of fluid.

Please tell him this:

On a 7 year old car it is unlikely the CAT or Sensors are faulty and need replacing. Unless there has been some serious neglect and damage.

Some of the most common reasons for P0420 are when the exhaust system after the CAT but before the lambda sensor has leaks from damage or corrosion which lead to air leaking in and upsetting the O2 sensor.

Get eye protection and a spray bottle and spray it with dish soap and water and look for bubbles where the exhaust has leaks.

Exhaust leaks can be repaired by an exhaust repair kit, often using high temperature sealants and clamping or tying metal sheets over the holes.

Another common reason is when ignition plugs or coils fail. Spark plugs are usually rated to well over 50k miles if they are iridium but you should check them.

Ignition coils will cause the engine to either misfire or not properly ignite which will lead to the code. It may be difficult to identify which plug is failing without an error code.

Everything mentioned here are not expensive to purchase and isn’t too difficult of a job to do.

how old is this vehicle? what is the model, mileage, and age?

unlikely it was ever the cat in my opinion.

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a common cause of P0420 is holes in the exhaust allowing air to leak in and upset the O2 sensor.

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get a spray bottle of dish soap and water, jack the car up, spray it while the engine is running and look for holes in the exhaust.

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another cause of P0420 is a bad ignition coil.

it’ll either clear itself or you can go ahead and do that. sure, what could go wrong.

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i personally suggest using an OBD tool to clear it up if you have one.

go ahead and get a new ignition coil and replace coil 4 and see what happens.

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coils are usually quite easy to replace and aren’t expensive.

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another way you could test it is to swap coil 3 with coil 4, and see if the issue moves.