Toyota Repair: air fuel ratio sensor, air fuel ratio, high mileage

Question
I took my 2002 Highlander to the dealership because it was running rough and the check engine light was on.  The DTC indicated trouble codes P171,P174 and P1130.  Also an exhaust leak was noted in the gasket just before the rear cat. convertor.   The dealer recommended replacing two AFR sensors at a cost of about $600 and fixing the leaky gasket for $145.  I have read that a leaky gasket could cause those trouble codes.

I am wondering if I should get the exhaust fixed and see if it clears up the problem before spending money on the AFR sensors?

Answer
Yes, absolutely, the 171 and 174 codes are directly caused by the exhaust leak, since you mentioned two A/F sensors I assume it's a V-6, in that case the 1130 code is for the bank one sensor one (B1S1) which is located in the exhaust manifold near the firewall, although they are a common failure I would stiil wait and do the exhaust leak first, clear all the codes and see if they come back,if anything the A/F failure might but if it's only one code then only one sensor should be replaced, the only reason I can think of that the dealer may want to replace both is because of high mileage.