I think that error code means that AHT can't be located. You need to run AHT from the 10.6 Applications Install Disc 2 that came with your iMac.
Here are some instructions:
support.apple.com/en-us/HT201257
Here are some step from the above link:
Using Apple Hardware Test on computers with OS X v10.7 and earlier:
1. To start AHT on these computers make sure the Applications Install Disc 2 is inserted into the built-in optical drive

2. Shut down your computer. Then, press the power button to turn it back on.

3. Press and hold the D key before the gray startup screen appears.

4. It takes a minute or so for AHT to start up and inspect your hardware configuration.

5. When the process is complete, select your language and click the right arrow. If you aren't using a mouse, you can use the up and down arrows to select a language and then press the Return key.

6. The AHT console appears. You can choose which sort of test or tests to perform:
To perform all of the basic tests, click the Test button or press the "T" key or the Return key.
To perform a more thorough diagnostic test, select the "Perform extended testing" checkbox under the Test button before you click the Test button.
Note: Your test results will appear in the window in the bottom-right of the console.

7. To exit AHT, click Restart or Shut Down at the bottom of the window.

Apple Diagnostics: Reference codes:
support.apple.com/en-us/HT203747

Slažem se, nemam CD koji mi je potreban. Ali isključivanjem vanjske grafike riješio se problem ali opet treba vidjeti što to točno nije ispravno.