check engine light on but no error codes

A couple of weeks ago the check engine light came on and when I had the codes checked it showed a problem with the EGR valve. I replaced it and the light went off. A week later the check engine light came back on but when I had the error codes checked there weren't any. After a few days the light went off but earlier today it came back on. There are still no error codes. Any ideas on what the problem is?

*** edited to add ***

I forgot to mention that it's a 2000 Camry, 6-cyl. I hope that helps.

There might be an intermittent fault. I would keep driving it and wait for the code to reappear.

Good luck,
Sam

Pull and put back the ECM fuse in the box by the battery. That will clear the CEL. Check for codes every time it comes back. If it persists with no codes, check the ECM under your glove box to see if all the connections are tight, and if it is bolted tightly to the chassis. May be a bad ground.

The light came back on. I got 2 P0128 codes and a P0401 code. I know the P0401 code is the EGR valve. I replaced that a couple of weeks ago. The P0128 code is "Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)" according to the FAQ. So I have to replace the thermostat to fix it?

Most likely you need to replace the thermostat, but there may be other causes. See below

For P0128 check the following:
Low engine coolant level
Leaking or stuck open thermostat <= Most likely if if coolant level is ok. Replace thermostat.
Faulty engine coolant temperature sensor
Engine coolant temperature sensor harness has a short or open circuit
Engine coolant temperature sensor circuit has a poor electrical connection

For the P0401 code, check the following:
EGR Vacuum Switching Valve (VSV)
Open or short in VSV circuit for EGR
EGR valve position sensor open or short circuit
Vacuum or EGR hose disconnected
EGR valve position sensor function