diff --git a/src/shearwater_predator_parser.c b/src/shearwater_predator_parser.c
index 81f6826e..908dddb7 100644
--- a/src/shearwater_predator_parser.c
+++ b/src/shearwater_predator_parser.c
@@ -680,12 +680,26 @@ shearwater_predator_parser_cache (shearwater_predator_parser_t *parser)
 			nsensors++;
 		}
 	}
-	if (nsensors && nsensors == ndefaults) {
+	if (nsensors && nsensors == ndefaults && !pnf) {
 		// If all (calibrated) sensors still have their factory default
 		// calibration values (2100), they are probably not calibrated
 		// properly. To avoid returning incorrect ppO2 values to the
 		// application, they are manually disabled (e.g. marked as
 		// uncalibrated).
+		//
+		// Submersion patch (Shearwater PNF O2 cells): restricted to the
+		// legacy layout this heuristic was written for. The Petrel Native
+		// Format records an explicit per-cell calibrated flag in the same
+		// byte read above, and a Petrel 3 sets it for all three cells while
+		// writing 2100 for each of them -- so on PNF the default value says
+		// nothing about whether the diver calibrated, and suppressing on it
+		// discards cells the computer itself reports as calibrated. 2100 is
+		// also the factor the computer uses: it displays ppO2 1.3 where the
+		// cells read 62/65/62 mV, and median(62) * 0.021 = 1.302. Verified
+		// against the log in test/native/fixtures/petrel3_ccr_o2_cells.bin,
+		// whose cells track the computer's own voted ppO2 within 0.05 bar
+		// across the whole dive. See
+		// packages/libdivecomputer_plugin/patches/0005-shearwater-pnf-o2-cell-calibration.patch.
 		WARNING (abstract->context, "Disabled all O2 sensors due to a default calibration value.");
 		parser->calibrated = 0;
 	} else {