Have your mechanic check your egr valve and port to make sure it is working.
If that checks ok and you still are failing NOX then your cat is bad. Any shop with a scanner should be able to tell if your cat is working. They should be able to read all your o2 sensors and verify that the cat is working properly. i would try another repair shop. It does not sound like this smog tech has much experience.
I agree a scanner would make some troubleshooting a lot easier. But once again, the vehicle is a '93. Where would you suggest they hook up this scanner? And given the very rich condition, even if they could read the [narrow band] O2 sensor, how would that determine how effective the cats are?
A rail leak down is similar to a cylinder leak-down test, but it tests the fuel rail. If you have a leaking injector it will tell you, though it won't pinpoint which one(s). You clamp off the return line and pressurize the rail. If it doesn't hold pressure you have a leak.
The water temp sensor is tested using an ohm meter at 2 or 3 temps, and compared to the temp-resistance chart in the manual. If the sensor is off the ECU may think the engine is cold and run rich. I think it's likely you could do a single test at room temp without removing the sensor and find out enough. Testing it at the ECU is best, as you also test the harness connections to the sensor at the same time.
What about the O2 sensors? When were they installed relative to the first failed smog test?
Cats could indeed be bad, but I would bet a roast beef sandwhich that a properly-running car would give better numbers with no cats at all. Plus, I think it's very unlikely the new OEM cats you put in were bad when you bought them. Failing all 3 compounds is somewhat rare for a smog failure. Usually HC and CO, or NOx.
How does the car run? Smooth? Does it misfire? What's the idle RPM?
