Check Engine Light & Heater Core

My wife has a 95 Tercel. It's been a good car (180,000+ miles) considering she drives like she's at Indy! Anyway, the check engine light is on. We had the EGR valve replaced and 20 miles after leaving the shop, the light came back on. My friend, who worked on it, is stumped. We had it tuned up, replaced spark plugs, wires, fuel filter, timing belt, etc. We don't want to take it to the dealership b/c we know they will charge some horrendous amount. Anyone have any ideas? Also, the heater core is going (it's leaking under the the dash on the passenger side. We've been told it's going to cost anywhere between $600 & $700 to replace b/c the part is aroung $200 or so and it will take up to 6 hours to replace. Does anyone know if this is the norm? The KBB value of the car is around $1500 and we don't want to put in a lot of money for nothing. We don't want to get rid of it b/c it's paid for and it's so good on gas. We just moved further away from her job and with gas prices, good gas mileage is a must.

Did you replace the EGR valve yourself? Were the two ports going into the intake cleaned out REALLY good? My 92 has 180k plus and the ports were completly clogged up. I had to ram a screwdriver in there and spray carb cleaner just to clear them out. One way to find out if they are clogged is to take the two nuts off the EGR and remove it, then start the car and if it runs fine the ports are clogged. If it runs like crap the ports are not.