That is an EGR Code. See
http://www.obd-codes.com/forums/1485-obd-codes-p0401---egr-flow-insufficient-on-
1996-toyota-corolla.html
or http://tinyurl.com/3d5jzw .

Regards,

Ed White

That problem related to the EGR system, which is controlled both via vacuum
and electronic valve actuation. This can be a tough code to diagnose
because there are multiple pieces in this system that can cause
insufficient gas flow. There is a very prescribed way to test it and
actually determine what the root cause is without just throwing parts at
it, and it makes it all the more difficult to do without Toyotas scan tool
(in other words, at a place other than the dealer).
 Whether or not it was caused during tbelt replacement is impossible to
say. The EGR system is not in any way related to the timing belt job, but
whos to say what happened, you know? Hell, I was driving a customers car
for a wheel vibration complaint one time several years ago, and the engine
decided to blow a connecting rod out through the block while I was in it.
It sure wasn't related to balancing wheels, but try telling that to the
customer who now has a 3000 lb paperweight instead of a car.