Reply To: Failed emissions test – think my cat is clogged but not sure

[joldancox]

The absence of a check engine light with a partially clogged cat is actually not that unusual because the oxygen sensors can still read within acceptable range even when backpressure has built up enough to cause the sluggishness and economy drop you are describing. My Altima threw no codes either until the blockage became severe enough to affect sensor readings which happened about two months after the sluggishness first appeared. Getting it replaced before that point saved me from additional issues that a completely failed cat tends to cause downstream in the exhaust system.