catalist below threshold code po420 97 merc grand marquis 4.6?

Question

engine ligh on steady

Answer 1

The catalist sensor could be bad. They are called the downstrean O2 sensors. The catalist convertor may be bad, the best way to diag this is have a Ford dealer go through the step by step tests.

Answer 2

usally means the cats are bad and need to be replaced.but take it to someone u trust whose a mechanic and have them test it fully to make sure there arent any other things wrong also

Answer 3

well sounds like a bad catalytic convertor and its not doing its job