Hi, we will need further information to completely answer this question. The 2004 Isuzu Ascender was available with either a 4.2L or 5.3L engine; at least for the 4.2L, the catalytic converter is available under two different part numbers depending on wheelbase; and we also need to know if the vehicle requires Federal or California spec converters. It would be best for you to call us at 800-505-3274 and speak to a sales associate.