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2017 Cadillac ATS 2.0T - Check Engine Light with P0420 Code

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Brittany E. · rep 0 · May 25, 2:49 AM · 1 replies · solved · Question

Hey everyone,

My 2017 Cadillac ATS 2.0T, with about 85,000 miles, just threw a Check Engine Light. I hooked up my OBD-II scanner, and it's showing a P0420 code: 'Catalyst System Efficiency Below Threshold (Bank 1)'.

The car seems to be running fine, no noticeable loss of power or strange noises. Fuel economy might be slightly worse, but it's hard to tell for sure. I'm trying to figure out if this is something I can tackle myself or if it's a definite trip to the dealership.

Has anyone experienced this specific code on their ATS? What was the fix? I'm hoping it's not a super expensive catalytic converter replacement right off the bat.

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 25, 2:49 AM

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ClearTheCode guidance (Cadillac · Diagnostics & codes)

Summary: For 2017 Cadillac ATS 2.0T - Check Engine Light with P0420 Code, work through these steps on your 2015+ vehicle:

  1. Confirm the symptom — note when it happens (cold start, highway, after fuel fill, etc.).
  2. Scan codes — save all pending/stored DTCs; don't clear until you've recorded them.
  3. Check TSBs & recalls — look up your VIN for open campaigns and known fixes.
  4. Inspect the obvious — related fuses, connectors, fluid level, and recent repairs.
  5. Test the common fix — follow model-specific service procedures; one change at a time.
  6. Escalate if needed — burning smell, no brakes, overheating, or stalling in traffic → shop visit.

Owner reported: Hey everyone,

My 2017 Cadillac ATS 2.0T, with about 85,000 miles, just threw a Check Engine Light. I hooked up my OBD-II scanner, and it's showing a P0420 code: 'Catalyst System Efficiency Below Threshold (Bank 1)'.

The car seems to be running fine, no noticeable loss of power or strange noises. Fuel economy might be slightly worse, but it's hard to tell for sure. I'm trying to figure out if thi…

This is educational guidance, not a substitute for hands-on diagnosis.

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