My AEM gauge shows a code (e.g. "0400") and then the display turns off. What should I check?

Power the gauge with the sensor cable disconnected and see how it behaves. This tells us whether the problem is in the gauge itself or in the sensor/harness. Also check that the gauge has a stable 12 V switched supply and a good ground. Send us the result (a short video helps) so we can advise on repair or replacement.