SPN 729 — Intake Air Heater Driver #1
Safe to driveEnough to reach the yard
Updated: 2026-11-14
J1939 code SPN 729. Below — how the fault is worded at the factory by Cummins ISB, DAF, Dongfeng, KAMAZ, KAMAZ-5490, MAZ, Renault Trucks and YaMZ, and what the second half of the code (FMI) means in the standard.
How to check it yourself
The output to the intake air heater — a powerful heating element in the manifold, switched on through a relay before starting in frost. This code is about the relay control circuit or the heater itself.
What it should read
| Heater current | hundreds of amps — hence the relay |
| Heater resistance | a fraction of an ohm — measure with it disconnected |
Check order
- Switch the ignition on with a cold engine: the heater relay clicks, and the 'wait to start' lamp on the dash lights (SPN 1081).
- The relay does not click — the control wire from the ECU, the relay winding.
- The relay clicks but the heater stays cold — the relay's power contacts, the thick wire to the heater, the heater itself (broken element).
- Power terminals on the heater: oxidation heats the terminal and melts the insulation.
Common causes — from most to least frequent
- Burnt contacts of the heater relay.
- Oxidised power terminals.
- Burnt-out heater.
- Relay winding or control wire.
Cummins ISBDAFDongfengKAMAZKAMAZ-5490MAZRenault TrucksYaMZ
Cummins ISB 6.7 engine
| FMI 3 | Engine intake air heater 1 circuit voltage high |
| FMI 4 | Engine intake air heater 1 circuit voltage low |
| FMI 5 | Engine intake air heater 1 circuit current low or open |
DAF
| FMI 3 | Preheating relay G014 short to power supply/open circuit at ECU pin B54 |
| FMI 4 | Preheating relay G014 short to ground at ECU pin B54 |
Dongfeng
| FMI 3 | Fan control output short circuit to battery |
| FMI 4 | Short circuit to ground of the heating relay output |
KAMAZ engine (J1939)
| FMI 3 | Air heater power stage: short circuit. |
| FMI 4 | Air heater power stage: short circuit. |
KAMAZ-5490 (Mercedes, K-line)
FMI is per the block maker's own table, not the J1939 table below.
| FMI 3 | Intake air heater driver: short circuit or open circuit. K-line code on KamAZ-5490: ADM3 14003. |
| FMI 4 | Intake air heater driver: short to ground. K-line code on KamAZ-5490: ADM3 14004. |
| FMI 5 | Intake air heater (PLD/MR2): open circuit. K-line code on KamAZ-5490: MR2 16409. |
| FMI 12 | Intake air heater (PLD/MR2): circuit fault. K-line code on KamAZ-5490: MR2 16432. |
MAZ
| FMI 3 | Intake air pre-heater driver circuit fault - Short to battery positive. Blink code: 3-2-1. Repair: Check the air preheater and its wiring. |
| FMI 4 | Intake air pre-heater driver circuit fault - Short to ground. Blink code: 3-2-1. Repair: Check the air preheater and its wiring. |
Renault Trucks
| FMI 11 | Intake air preheating: VECU protection not operative. Repair: Check the continuity and insulation of the wiring (SL2/9), check relays R15 and R16. |
YaMZ
| FMI 3 | Intake air preheater driver circuit fault (short to battery positive); or: Air preheater No. 1 test error (blink code 3-2-3). Blink code: 3-2-1. Repair: Check the condition and connection of the air preheater. |
| FMI 4 | Intake air preheater driver circuit fault (short to ground); or: Air preheater No. 1 test error (blink code 3-2-3). Blink code: 3-2-1. Repair: Check the condition and connection of the air preheater. |
What FMI means in J1939
Can I keep driving
Safe to drive. Below — how much time you have and what happens if you keep going.
What happens if you keep driving
Boost is not reaching its target: turbo, intercooler or intake. The engine compensates with mixture and runs hotter than usual.
You will lose pull, burn more fuel and add overheating on long climbs. If you hear a whine or a whistle from the turbo, stop — the wheel is breaking up.
This is an electrical fault: an open, a short or a bad contact in the sensor circuit. The value it measures is most likely fine. You can drive. But until the circuit is repaired this system runs blind and cannot warn you about a real failure.
What to check on the spot
Inspect the air filter and charge-air hoses: leaks and cracks produce exactly these codes.
Nearby: intake / boost
- SPN 51Engine throttle valve position
- SPN 102Intake manifold pressure (boost)
- SPN 105Intake manifold air temperature
- SPN 106Engine Intake Air Pressure
- SPN 107Air filter differential pressure
- SPN 132Intake mass air flow
- SPN 411EGR Differential Pressure
- SPN 730Intake Air Heater Driver #2
- SPN 2791EGR Valve Control
- SPN 3464Throttle Actuator Control Command 1
Makes with this code
KAMAZMAZYaMZDongfengDAFRenault TrucksCummins ISBKAMAZ-5490
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