Produced from 1979 to 2002, Mazda's 626 didn't exactly debut as the success it would later become. It wasn't until Ford and Kia got involved in 1983, marketing the 626 as the Telstar and Concord, respectively, that the 626 got the worldwide attention that it deserved. Of course, Mazda's well-known reliability and chassis tuning helped, but a couple of turbos really sealed the deal. The introduction of Onboard Diagnostics, Series II protocol in 1995 helped Mazda's fun family car meter and monitor air and fuel even more accurately, while sending up red flags when something went awry.
Code P1506 is a manufacturer-specific code -- as denoted by the "1" in 1506 -- indicating that engine speed at idle is 100 rpm lower than it should be for 14 seconds or more. The 626 modulates idle speed in three ways: by adjusting the amount of fuel injected, the amount of air flowing through and spark timing. The 626, like many cars, used an idle air control valve to meter the appropriate amount of air into the engine at idle. If the idle is lower than it should be with a given amount of air, fuel and spark, the computer will trip a code P1506.
The idle air control valve is a computer-controlled valve that opens and closes to allow air to go around the main throttle blade, so the engine doesn't have to rely on a set throttle position to meter airflow at idle. The channels that carry air around the throttle blade are fairly small and can collect dust, dirt, oil and carbon over time. When dust combines with oil -- possibly from a bad positive crankcase ventilation valve -- it will form deposits in the channels that block airflow. The computer will open the valve more to compensate for blockage, but the valve can only open so far; eventually, airflow will reduce to nothing.
Blocked passages will gradually choke your idle down to nothing, but a malfunctioning IAC valve will kill it outright. IAC valves can malfunction just like anything else on your car, and if you're receiving code P1506, yours may not be working as it should. The IAC doesn't sit still when it opens; it continuously adjusts position to maintain idle. Over time, the IAC's reaction time may slow to a crawl, causing the idle to drop or surge. Of course, even a properly functioning IAC won't do much good if it's not getting enough air. An excessively clogged air cleaner will drop idle rpm as well, though it would have to be so clogged -- or wet -- that the car would barely run at anything over idle.
Your car's A/C compressor draws about 8 horsepower from the engine; running the A/C will naturally drag your car's idle down by a couple hundred rpm. The computer will normally ignore the rpm drop or raise rpm by opening the IAC and injecting more fuel -- but only if it knows that the A/C is on. If you have a malfunction in the A/C compressor relay circuit, the compressor will run without the computer knowing it and will drop rpm. Though, that assumes that the computer is doing anything right; almost any kind of failure in the computer itself will manifest as something weird at idle, possibly low rpm and a code P1506.
