My 2003 Forester idles at 5000+ rpms after being stuck in traffic

there are sensors that input ambient temperature , barometric pressure and mas air flow into the ECM so that the system gets optimum air fuel ratio for a start and to keep the cold engine running
(Just like having to apply the old choke system)
normally the engine will idle at around 1500 rpms for a few minutes until the input sensors read a change and the ECM makes the necessary changes to the fuel settings and the rpms as dictated by the now in use HO2S sensor in the exhaust
However if you feel that the idle rpms is far too high and for too long then have the fault codes read as there may be a faulty sensor in the system

usually an O2 code is 3 digits and not this type of code. if it has an mass air flow sensor(MAF) it could be it. I would start with checking the MAF readings then the O2 readings. OBD 2 codes are emission regulated so no matter what car it is the codes represent the same problem.

Spark Plugs --change every 3 years?
Front O2 Sensors-replaced at 100,000 miles?
MAF Sensor-cleaned?
Egr System works as it should?
No cylinder misfires?
No Vacuum Leaks anywhere?
No stored codes?

Shall I keep going?

You have to work on all vehicles at least twice a year
& look for problems

Alternator has no ac leakiage
Fuel filter new
Fuel pressure good
Air filter?
Throttle Plate doesn't stick?

symtoms of a bad coolant sensor. poor idle, stalling, rich mixture, retarded timing, low idle speed, cold hesitation or stumble, poor fuel milage. ( will also ****** timing ). in some cases you can unplug sensor and the pcm will compensate by running a default air/fuel ratio and in some cases allowing car to run without the sensor.Id just replace it !! if light is on, get vehicle scanned to find problem code. most all auto parts stores will scan for free, auto-zone etc. have a good day !!

If the clutch is faulty you will have a poor pick up as the clutch might slip or make a shuddering noise while it engages all the time.
The fault seems related to idling settings on the fuel pump, check the throttle sensor check for fuel lines, fuel filter and fuel pressure- needs a calibration.

Find your vacum leak. It is sucking in to much air.

idle air control valve sensor and a tune up would probably help the fuel milage plus a new 02 sensor. catalystic converter may be getting restrictive as well. new fluids may help for milage as well.

It is actually hard to say without actually seeing the vehicle, but the target speed for your engine idle is around 750 RPM. The stalling after revving kind of depends on how high you are revving it and whether the engine is at operating temperature or not. This may be normal. Then again, it could be that the Idle Air Control (IAC) is not working properly.

The shifting from first to second at 4000 RPM sounds like a completely separate problem. This could be due to low transmission fluid level, sticking shift control solenoids, or possibly even a malfunctioning Vehicle Speed Sensor.

If the Check Engine light is on, have that checked first.

M.a.P. sensor, air leak in intake system, low or erratic fuel delivery

map sensor