Dealer Changed plugs and wires and now runs rough??

I have a 2012 Avalanche that the dealer just did the plugs and wires on as well as a fuel system cleaning. It is now running more rough compared to before. Even more so when cold and gets a little better when warmed up, but still more shake compared to previously being serviced. No codes are showing as i took it back and they did not see any issues. I drove it a few weeks and took it back again as it is quite a bit different in the smoothness. Any thoughts on what would cause it and not show up on codes or their computers? They have it again now and i am going to go and drive it with them to better explain.

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[Richard KirshyParticipant]

Unless some other issue has arisen.
Could be faulty gaped plugs
Incorrect plugs
Incorrect wires
When you say fuel system cleaning do you mean injector cleaning or from the fuel tank forward?
One other item to note:
Is this a flex fuel vehicle? if so have them make sure the sensor is reading the correct amount of ethanol being used.

• This reply was modified 3 years ago by  Richard Kirshy.

[Charles CravenParticipant]

this is not a Flex. They did the injector cleaning i believe. If a incorrect plug, wire or bad gap, would it not show? The computer shows it firing.

[Richard KirshyParticipant]

The on board diagnostics will show an issue providing the issue is out of the operational parameters.

[Author]

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