diff --git a/display-board-1408.md b/display-board-1408.md
index 356757d6..0589704b 100644
--- a/display-board-1408.md
+++ b/display-board-1408.md
@@ -55,9 +55,10 @@ J1 (14-pin, from the RIO's EXTERNAL DISPLAY port) is all input:
 ## What the RIO firmware displays (v4.2)
 
 All display writes go through `$CB49` (buffer render) or five canned
-patterns at `$CB7C–$CC52`, driven from three contexts: initialization,
-the built-in **test mode**, and the **serial error/crash displays**
-(below). During normal, healthy gameplay the display is static.
+patterns at `$CB7C–$CC52`, driven from four contexts: initialization,
+the built-in **test mode**, the **serial error/crash displays**, and
+the **CheckRequest self-test** (all below). During normal, healthy
+gameplay the display is static.
 
 **Normal operation:** `F0000000` — written at cold boot (`$C068`) and warm
 re-init (`$C64B`), then left alone.
@@ -92,6 +93,20 @@ In the axis tests `hhll` is the **live 16-bit quadrature count in hex**
 and watch the number move. The channel IDs are literally the keypad keys
 that start them, which is why the fourth axis reads `0C`.
 
+## The CheckRequest self-test leftover: `04000000`
+
+The host `CheckRequest` (`0x80`) triggers a full self-test in the
+handler at `$C5A6`: it sets the same display flag test mode uses
+(`$2421`), brackets itself with `TestModeChange 0x8C` notifications,
+flashes a lamp pattern, scans the pods, and sweeps the five encoder
+channels — rendering each channel's `cc00hhll` frame as it goes. It
+clears the flag on exit but **never repaints the display**, so after
+every status check the digits are left showing the last frame:
+channel 4, usually `04000000` with idle encoders. It is a stale
+snapshot (axis movement does not update it — bench-confirmed
+2026-07-19), purely cosmetic, and the board keeps operating normally.
+Any reset repaints `F0000000`.
+
 ## Error displays (the third use)
 
 Two more display writers live in the serial machinery — the display is