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Troubleshooting SMI Air Force One Braking System Triggering Check Engine Light on Toyota Corolla


Approximately two years ago, I installed the SMI Air Force One supplemental braking system, Model AF0115 for Motorhomes with air brakes. I now appear to have a problem with the Operating Unit mounted in the towed vehicle 2017 Toyota Corolla iM w/manual transmission. I procured the AFO system via etrailer. This past week, the Engine Check Light came on with an OBD code P0171 Lean Bank 1. It was attributed to a continuous, and very noticeable, vacuum leak on the threaded exhaust port on the AFO Operating Unit. I attached a picture .pdf format for reference. This diagnosis was performed by the Toyota dealership on 12/26/19, in Auburn, MA. The dealership put a vacuum cap on the port, and subsequent OBD check showed no P0171 fault. Per AFO installation instructions, this port should be left OPEN, unless the vehicle is equipped with Hydro Boost brake system. The Toyota Corolla iM does not have a Hydro Boost brake system. Requesting clarification, as the port is currently capped.


Expert Reply:

I reached out to my contact at SMI (now Demco) and he said that on your 2017 Toyota Corolla iM the exhaust port on the Air Force One # SM99243 operating unit does need to be left open.

He said the issue in your case is that the vacuum check valve between the tee and operating unit has likely failed. You can simply replace this vacuum check valve with part # SM99909 and face the black portion of the valve towards the unit, making sure to unplug the exhaust port in the process.

This should resolve the issue you had with the P0171 fault.

expert reply by:
Chris R

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