P0300, P0171, P20EE, P0420 real 2026 shop ranges, urgency and first tests.
70%+ of mystery downtime starts as ignored dashboard lights. P20EE derates within 50-100 miles. Real invoice aggregates, not textbook tables.
This guide covers the general case. For a report tailored to your exact symptom, vehicle, and urgency, run the free AI triage — it takes under two minutes and gives you a printable playbook for your driver or shop.
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