One problem related to underhood wiring has been reported for the 2003 Toyota 4Runner. The most recently reported issues are listed below. Please also check out the statistics and reliability analysis of the 2003 Toyota 4Runner based on all problems reported for the 2003 4Runner.
After replacing an oxygen sensor several times, the dealership finally discovered the root of the problem that is making the oxygen sensor and traction control circuit short out. The air divertor flap inside the hvac box under the dash had grease applied to the flap hinges in the factory. Directly below the flap is a wiring harness which drops down from the hvac box and plug into the ecu. Over time the grease has traveled down through the wiring harness to the electrical connection plug into the ecu causing various circuits to short. No damage at present, however various safety circuits such as traction control are being affected. Furthermore, as oil is combustible, an interior fire could result from the oil penetrating the 12 volt electrical connection point on the ecu. This issue/ problem should be considered hazardous and should be a recall to replace the wiring harness and ecu as well as clean the grease out of the hvac box.