Hi,
The codes are both related to the oxygen sensors, as I believe you already know. The thing is, the downstream code (P0141) is likely the result of the P0135.
The possible causes are broad. The only way to determine the issue is to go through the diagnostics provided by the manufacturer. They are extensive, but that doesn't mean you will need to complete all of them. LOL, You may find the cause within the first few tests.
I attached all the diagnostics below. Take a look through them and let me know if you are comfortable performing them.
Let me know.
Joe
See pics below.
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Saturday, September 23rd, 2023 AT 8:35 PM