Volvo D13 Intermittent SPN 3221 FMI 5 - NOx Sensor Intake Open Circuit

Question

My 2018 VNL started throwing an intermittent check engine light with a derate warning. The dash is showing SPN 3221 FMI 3 and FMI 5, coming and going every few hours. Tech Tool shows it as the intake NOx sensor circuit, but the sensor was replaced only six months ago.

obd2_owl · Accepted Answer

Found the issue at the chassis harness breakout near the starter. The pink power wire for the NOx sensor (721H) had a tiny nick that corroded green, causing it to lose continuity whenever the truck hit a bump. Cut out the 4-inch section of bad wire, soldered in a new 16awg lead with heat-shrink, and re-secured the loom away from the heat shield. Cost was just an hour of labor and some shop supplies, and the SPN 3221 has stayed inactive for 5,000 miles now.

hd_trucker_77 · Answer

Check the power supply at the NOx sensor connector pin 1. Those Volvo harnesses are known for rubbing through the loom right behind the sleeper or near the transmission bellhousing, which can cause these intermittent open circuit codes.

Jake Morrison · Answer

If you are using aftermarket NOx sensors, that is likely your problem. The Volvo EMS is extremely sensitive to the resistance values on the heater circuit of the sensor. I always recommend sticking with the genuine Continental/Volvo parts even though they are double the price.

Sam Holloway · Answer

Since you have FMI 5 (current below normal or open circuit) and FMI 3 (voltage above normal), it sounds like a data link or power ground issue. Check the CAN resistance at Pins 3 and 4; you should be seeing 60 ohms with the batteries disconnected.

Volvo D13 Intermittent SPN 3221 FMI 5 - NOx Sensor Intake Open Circuit

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