How to fix P0403?

P0403 is an OBD-II code that indicates a malfunction in the exhaust gas recirculation (EGR) system. The EGR system reduces nitrogen oxide emissions and cylinder temperatures by recirculating exhaust back into the engine.

I just had this problem on my Highlander. The EGR system is a closed system. Leaving the gas cap loose or a defective cap will cause that DTC.

In my case, a squirrel decided to dine on the hose and opened it up the system. Because it was open for a long time, it sucked dirt into the system and caused other parts to fail. The hose on the Highlander is part of the gas tank, so the tank needed to be replaced along with a pump and a valve. Fortunately, I have comprehensive insurance which paid for it - about $8000. altogether.