What is wrong with my honda odyssey the Malfunction indicator lamp is on and the engine is reving up and down in park or netural but idles normal when in gear? The TCS light is also on for no reason.

Hello. It seems your Honda's alarm system is malfunctioning/defective. The 'parking lights' are supposed to flash on/flash off when arming/disarming the car alarm. If the parking lights stay lit; that indicates a 'short circuit' somewhere between the alarm and 'parking lamp relay'. Good luck and keep me informed; if you want to.

sound like the idle air control sensor IAC it could be failing .

newer honda's have dash cluster computer. "CAN BUS" type.

so list which one,s.. Oil, CEL, charge,,? name a few. or all.
also there are parking lamps, are they stuck on too?
some newer I/P clusters have test mode, did you read the
glove box operator guide, ? it tells of all that.... if true.
did the engine stall, this is normal for all stalls.

close the doors and start engine
the cluster lamps all glow key on , not started engine,that is normal.
tell the full story.
i open car door, sit in drivers seat
close door.
then.... you see what?
then key on
see lamps glow, that is normal.
start engine, see them mostly out,normal.
the lights not out, now,are called warnings.

RTM? or Read the meticulously formulated manual RTMFM.
see here. read , the section on, before driving...
http://justgivemethedamnmanual.com/honda/honda-odyssey-owners-manuals/

Thats the Traction Control System...indicating one wheel is spinning at a different speed than another.

It just so happens that the ABS Braking System ties into this as well.

I'd check the brake fluid level first...then the fluid level switch (some are in the cap and some are in the brake master cylinder.)

Hope this helps...

Not sure I know the main concern here

What was the steering issues your working on?

The PO505 is a Generic Code -for Idle Control
System Malfunction

The following is from the Honda Odyssey (1999-2004) Service Manual page 19-81:

"When the Malfunction Indicator Light (MIL or check engine light) and the Traction Control System (TCS) light are both on at the same time, you should troubleshoot the Programmed Fuel Injection (PGM-FI) system first."

"The TCS indicator will come on if one or more of the following components are faulty:
heated oxygen sensor, manifold absolute pressure sensor, crankshaft position sensor, engine coolant temp. sensor, throttle position sensor, top dead center sensor 1 or 2, intake air temp. sensor, exhaust gas recirculation, barometric pressure sensor, idle air control valve, VTEC pressure switch, knock sensor, fuel supply system, automatic transaxle, and misfire detection system of the PGM-FI system."

Most major auto parts stores have a code reader that will read out your diagnostic trouble codes or DTC's that are stored in the car's computer when the MIL comes on. They will do it for free. With these codes, you can narrow down what is wrong. This is good information to have if you are dealing with a mechanic or dealership. The terminal to hook up the code reader is under the driver's side dash about in line with your right shin.

P505=Idle System Malfunction
This is probably a throttle positioning sensor problem.

OK, have the computer scanned to see what system or circuit caused the malfunction indicator lamp to light.

A good indication of a blown fuse,