Tech Tips (by ALLDATA): Subaru ‘dies’ on freeway

TECH TIPS (courtesy of ALLDATA)

Mileage: 62,009

Problem: The vehicle died while driving on the highway and was towed to the shop.

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Details: When the vehicle was brought into the shop, the engine would crank over but not start. The tech checked fuses and found fuse #17 in the engine compartment had blown. When he tried another fuse, it immediately popped – there was a dead short to ground. He connected a scan tool to pull diagnostic trouble codes (DTCs) and found that there was no communication with PCM, but he could communicate with the TCM. He pulled DTC, U0100 – Lost Communication with ECM/PCM “A”.


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The tech checked the W/R wire for a short circuit – it was good. He noticed that the main relay was not clicking on or off. After removing and reinstalling it, the relay began working properly.

Confirmed Repair: The tech replaced the main relay and #17 fuse, cleared the code and retested. The engine started right up and did not die on the test drive. When he checked again after the test drive for DTCs, there were none. Fixed.

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