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Just ran a scan on my car... 2004 Cadillac DeVille DTS (4.6…

Just ran a scan...

Just ran a scan on my car... 2004 Cadillac DeVille DTS (4.6 V8) all codes clear except P0420 (Catalyst System Efficiency Below Threshold). Also In correlation with this is the Evaporative System Monitor (?) test showed incomplete...!??! Help... first time Caddy owner here, love the car, but I don't want a lemon lol. Also... They did a coolant flush two days ago, and 1. they underfilled it. I think... (I hear a water sloshing noise in dash when I turn, and when I accelerate), and 2. they used regular 50/50 Universal Antifreeze instead of OEM Dexcool... Is this going to cause serious corrosion damage to the engine? Should I have them drain, flush & refill using Dexcool? Or will it be okay? I'm worried about that, and the code... The exhaust system was cleaned recently so I wonder if they may have damaged a sensor in the process? HELP! Thanks for your time!

Mechanic's Assistant: Which particular sensor are you asking about? And have any trouble codes been triggered?

P0420 Is there a sensor associated with this code?

Mechanic's Assistant: Are you fixing your DeVille yourself? What have you tried so far?

Nada... just bought it yesterday

Mechanic's Assistant: Anything else you want the mechanic to know before I connect you?

The car has 111k miles, other than the code & coolant issue, everything purrs like a kitten.

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Answered in 12 minutes by:
1/18/2018
Chris (aka- Moose)
Chris (aka- Moose), Cadillac Technician
Category: Cadillac
Satisfied Customers: 51,587
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  1. P0420 (Catalyst System Efficiency Below Threshold). This code means the bank 1 catalytic converter is no longer working efficient and needs to be replaced.
  2. Also In correlation with this is the Evaporative System Monitor (?) test showed incomplete...!??! This warning means the drive cycle monitors are not complete. Monitors are the computers way of saying all systems have been checked and are working, so it can then accurately control all systems. Anytime the battery voltage is removed from the car or a scan is done to clear codes or monitors then they all must complete again from normal driving use. This monitor takes the longest to complete. The fact it has not cleared means they likely knew the 0420 was there and they disconnected the battery to try and erase the code to hide it from you.
  3. They did a coolant flush two days ago, and 1. they underfilled it. I think... (I hear a water sloshing noise in dash when I turn, and when I accelerate), This would not be caused by coolant level. That would be caused by a water leak into the car and there is water in the channels of the body under the carpet, the cowl panel under the wiper is clogged with leaf debris and the water there can not drain off, or the evaporator case drain is clogged and evaporator condensation is sloshing around.
  4. they used regular 50/50 Universal Antifreeze instead of OEM Dexcool... Is this going to cause serious corrosion damage to the engine? Should I have them drain, flush & refill using Dexcool? Or will it be okay? Aftermarket coolant is fine as long as it has all the properties of the DEXCOOL.
  5. Now that I have posted a partial or complete answer, you will see you can rate me. The deposit you have already paid is not transferred to me until you rate me. I would like to receive nothing less than 5-star excellent ratings. So if we are not there yet, please reply and let me help more. If you're not pleased, let me know because rating bad does not result in a refund, there are better options. Rating now will not close this post nor prevent replying. Thanks Chris
Chris (aka- Moose)
Chris (aka- Moose), Cadillac Technician
Category: Cadillac
Satisfied Customers: 51,587
Experience: 16 years of experience
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Customer reply replied 4 months ago
Great! The cowl is an easier fix, than burping the coolant system. I'll check into it. However... Could the SES light go out after driving the car a few days giving the system time to run all system checks etc... Whats frustrating is the gentleman I bought the car from mentioned cleaning the entire exhaust system prior to my purchasing it... Maybe just hopeful thinking, but still... If I can prevent $500+ to replace the Catalytic Converter I'd be happy to hear some suggestions. Thanks again!

The SES is on for the 0420 code not for the EVAP monitor. That code can go away as well as the light intermittent but it will return. The converter has to be replaced to resolve that concern.

There are no permanent fixes for the converter with snake oil mechanic in a bottle that will work.

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Customer reply replied 4 months ago
Understood, thank you for your time! I will definitely come back with any questions I have about this car in the future. Ciao! :-)

You are welcome

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