It's a '92 corolla DX. I may or may not have a bad egr valve, but for sure the solenoid isn't doing anything. It's not switching no matter how warm or high-revving the engine is. I replaced the solenoid today, no difference. If I run vacuum straight to the end of the egr it seems to open, but the solenoid won't run any vacuum to it. Where does it gets the signal to turn on? Does it have a fuse? Does it run from a heat sensor? And why does my egr have an adjustment screw, I've never seen that before.

Camry 2000 2.2 LE with California emission.

CEL(check engine light) went on after driving about 10 minutes, but the car ran normally. I started to look for info on the web while still driving the car each day. A few days later, I went to AutoZone for a free code pull out. It was P0125 and P0401.

Checked the CHILTON repair manual:
For P0125, the identification is ‘insufficient coolant temperature for closed loop fuel control’. The possible problem area is ‘Bank one Air/Fuel sensor malfunction, or PCM malfunction.’

Since the engine temperature reached the middle of the gauge on the dashboard after five minutes of driving, I excluded the coolant temperature sensor, and got into the A/F sensor.

Test the A/F sensor:( please see Technical Article 37 at http://www.autoshop101.com/)

The bank one A/F sensor is located at the exit of exhaust gas right next to the engine and before the catalytic converter. There are four wires (two black, one white and one blue) on its round column end. Unplug from the other end of the cable (that is a square shape plastic connector). Use a DMM (digital multi-meter) to measure the resistance across two black wires (that is the heater of A/F). The reading should be 0.8 to 1.4 ohms. (You may short the DMM probes to get the short circuit reading first, then compare to the reading of the heater. The difference should be the real heater ohms). Mine is 1.3 ohms (that means the heater of A/F is ok). Plug back the connector, and back probe the heater with the engine key at on (engine not running) to measure the voltage. Mine is 10 volts (that means there is voltage apply to the heater). Back probe the sensor element (that is between the blue and white wires). Run the engine, the voltage was between 0.1 and 0.2 volts with different engine speed (I don’t know if it is right or not). After the engine cooling down, pull out the EFI (15A) fuse for a minute and put it back to reset the computer (NEVER reset the computer while engine is hot, the car loses it drivability due to very low idle speed on its re-learn cycle).
On the second trip, CEL was on again. The code was still P0125 and P0401. Replace the A/F sensor ($205.00+tax from dealer). Reset the computer. On the second trip, CEL went on again. The code was only P0401 this time (that means P0125 was fixed by replacing the A/F sensor)
Check the repair manual. P0401 means ‘EGR insufficient flow’. Possible problem is
‘EGR valve stuck close, or MAP sensor’
Since the car ran normally, I exclude the MAP sensor having problem.
What causes it? (Please see Technical Article 61 at http://www.autoshop101.com/ to learn how EGR valve works)
1) EGR valve is too dirty, and mechanically get stuck.
2) EGR valve cannot hold the vacuum due to its damaged diaphragm.
3) No, or not enough vacuum applied to the top of the EGR valve to pull the valve open properly.

Here is what I did:
1) Checked all hoses around EGR valve and EGR Vacuum Modulator for cracks. No cracks found.
2) Checked the engine vacuum at port E on the throttle body by a handheld vacuum tool (got it from AutoZone for $30.00) with engine running. It was 22.5 inch (should be between 17 to 25 inch). So, engine vacuum was ok.

3 ) Took the EGR valve out and check any blockage. No blockage found. Clean it with a brush anyway. Use two fingers parallelly push up the diaphragm and check if it open up the valve. Use the vacuum tool to apply vacuum to the top of the EGR valve, it started to open up when vacuum was about 2.5 inch. (If you don’t want to remove the EGR Valve, run the engine at idle and apply about 3 inch vacuum on the EGR vacuum port, the engine should stall. That means EGR works normal)

4) Clearly label all the vacuum hoses, then remove the EGR Vacuum Modulator, open the top cover and clean the filter there.

5) Then apply vacuum to port P while blocking ports R and Q, the vacuum should bleed out at the filter hole.

6) Now use a balloon pump (got it from 99 cent store, need a third hand) to apply some pressure at the bottom of the EGR Vacuum Modulator while repeating the step 5 above. The vacuum should stop bleeding from the filter hole and holds the vacuum there.

7) Apply vacuum to port P while blocking port R and the filter hole, the vacuum should bleed out at Port Q.
Apply vacuum to port P while blocking port Q and the filter hole, the vacuum should bleed out at Port R. (I figure it out this to test after brought a new EGR Vacuum Modulator. Too late)

8) Put everything together to the car except the hose to Port Q of the EGR Vacuum Modulator. Apply vacuum to that hose, which goes down to the VSV for EGR (vacuum switching valve for EGR) at the back of the engine, then comes back through the other hose to the top of EGR valve. With you finger to touching the diaphragm of the EGR valve, it should start move when vacuum is above 2.5-3 inch. (When engine is not running vacuum should be able to go through the VSV. When the engine is cool and running, no vacuum should go through the VSV. When engine reaches its normal temperature and running above 2000 rpm, vacuum should start go through the VSV). Put the hose back to port Q.
9) Start the engine, pull out the hose at the bottom of the Modulator. There should be some exhaust gas from the exhaust manifold blowing out. Increase the engine speed by move the throttle, the blowing out pressure should be increased too (if not, there must be blockage in the steal pipe under the EGR valve). Put the hose back.
10) Use the method from Technical Article 61 at http://www.autoshop101.com/ to check the low and high vacuum at the EGR. The low was 2.5 in, the high was only 4.5 inch (normal should be 3 and 13 inch)

Everything look is normal to me, except the high vacuum was too low. Reset the computer. On the third trip, the CEL went on again. AutoZone checked the code. It was still P0401. The only possibility is the VSV for EGR valve for someone on the Internet said there is intermediate problem on this VSV. (The CEL went off by itself once for about five days. After a 100 miles trip, CEL went on again with the same P0401 code. This was indeed an intermediate problem. )

11) Replace the VSV for EGR valve. It is at the back of the engine. I had to ramp up the car to reach it (that was why I put this job to the last). The part costs about $80. I didn’t reset the computer this time. When I started the engine for the fourth trip, the CEL went off itself (This matches the repair manual’s saying; if the problem goes away for three consecutive trips, the computer will turn the CEL off; if the same problem detected for two trips, the computer will turn the CEL on).

I can conclude that it was the VSV for EGR causing the P0401 code.
What I learn from above:
A) The EGR valve is a very simple part. As long as it moves when there is vacuum applied to it, and holds the vacuum, plus no physical blockage to be seen, it should work normal. That is why I insisted not to replace the EGR valve even though many suggested to do so (the part is expensive too, above $200)
B) The EGR vacuum modulator is simple too. As long as it passes the tests of step 5, 6 and 7 above, it should be working normally. When thing is simple, it is difficult to break down too.
C) Toyota intents to put the troublesome VSV in the hard to reach area to force customers to buy the unnecessary parts, or pay the dealer to do the service. Toyota parts are so expensive that selling parts makes ten times more profit than selling a car in Toyota. That is a very bad mentality. Think ten times before considering the next Japanese car.

I can be reached at hongfengdu@yahoo.com