Hi, recently had a similar problem.Towed our caravan back from Cornwall, the engine light came on, stopped for a few minutes started up light went out and stayed out until I had to put more power on an incline on the A30, light came on.Same problem as before so a careful drive home,no more light, but notice fuel consumption had increased.
Used the Alphard solo a few times after and same problem. Asked around and read Hand book which suggested oxygen/fuel ratio sensor may need replacing. Replaced it, the one before the 2nd Cat,much better, but still came on on occasions. Then the wife noticed a rattle from the exhaust, onto the 4 poster and found a rattle from the 2nd Cat.which sounded like the internals had disintegrated.Had a complete S/S exhaust fitted[Described and as shown in my recent post]. Couldn't fit a new after Cat sensor, as the new one I received had a wiring harness that was too short. When I get time I'll take off the old one and splice the old wiring onto the new wiring.
What I have found out since is that the 02-08 Alphards have 2 cats.One as part of the exhaust manifold,and one 1/3 of the distance from the front,this is because of the very strict polution laws in Japan.
Hope this helps.