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SPN 51 — Engine throttle valve position

Safe to driveEnough to reach the yard

Updated: 2026-11-14

J1939 code SPN 51. Below — how the fault is worded at the factory by Cummins ISB, KAMAZ-5490, MAZ, Meritor (ex-WABCO), Renault Trucks, SITRAK, YaMZ and ZF AS-Tronic, and what the second half of the code (FMI) means in the standard.

How to check it yourself

The intake throttle position sensor. On a diesel the throttle does not control power: it is closed to push exhaust into recirculation, to heat the particulate filter during regeneration and to shut the engine down more gently.

What it should read

Reference voltage at the connector, ignition on5.0 V ± 0.25
Signal in the working range0.5…4.5 V

Check order

  1. Remove the intake hose and look at the throttle: oil soot stops it moving and closing fully.
  2. Run the throttle test from diagnostics: it should travel from stop to stop without sticking.
  3. FMI 7 (not responding): soot or a jammed actuator; on a pneumatic actuator — an air leak to it.
  4. FMI 3, 4: the actuator connector and the 5 V reference.
  5. After cleaning or replacement, many makes need the end positions taught.

Common causes — from most to least frequent

  1. Oil soot on the throttle.
  2. Throttle actuator.
  3. Connector.
  4. End positions not taught.

Cummins ISBKAMAZ-5490MAZMeritor (ex-WABCO)Renault TrucksSITRAKYaMZZF AS-Tronic

Cummins ISB 6.7 engine

FMI 2Intake throttle actuator position sensor signal erratic or incorrect
FMI 3Intake throttle actuator position sensor circuit voltage high
FMI 4Intake throttle actuator position sensor circuit voltage low

KAMAZ-5490 (Mercedes, K-line)

FMI is per the block maker's own table, not the J1939 table below.

FMI 0Throttle actuator position: signal above normal range. K-line code on KamAZ-5490: MR2 12419.
FMI 1Throttle actuator position: signal below normal range. K-line code on KamAZ-5490: MR2 12420.
FMI 2Throttle actuator position: signal implausible (MR2 12424); Throttle actuator position: signal characteristic invalid (MR2 14217). K-line code on KamAZ-5490.
FMI 3Throttle position sensor: short to high. K-line code on KamAZ-5490: MR2 14205.
FMI 4Throttle position sensor: short to low. K-line code on KamAZ-5490: MR2 14206.
FMI 7Throttle actuator position: faulty. K-line code on KamAZ-5490: MR2 12431.
FMI 13Throttle actuator position: position out of limits. K-line code on KamAZ-5490: MR2 14218.

MAZ

FMI 2Intake throttle valve stuck - Valve/damper stuck. Blink code: 6-7-4. Repair: Check the intake throttle actuator.
FMI 3Intake throttle control valve fault - Control signal above allowed. Blink code: 6-7-3. Repair: Check the intake throttle control valve and its wiring.
FMI 4Intake throttle control valve fault - Control signal below allowed. Blink code: 6-7-3. Repair: Check the intake throttle control valve and its wiring.
FMI 12Intake throttle actuator error - General error. Blink code: 6-7-8. Repair: Check the intake throttle actuator and its wiring.
FMI 20Intake throttle position control error, sustained deviation - Trim coefficient drifted far from its initial value. Blink code: 6-7-5. Repair: Check the intake throttle actuator.
FMI 21Intake throttle position control error, deviation from target - Trim coefficient changed sharply within a drive cycle. Blink code: 6-7-9. Repair: Check the intake throttle actuator.

Meritor (ex-WABCO)

FMI 3Power Amp. Out Valve Right Rear — Shorted to Battery
FMI 5Power Amp. Out Valve Right Rear — Open Circuit
FMI 6Power Amp. Out Valve Right Rear — Short to Ground

Renault Trucks

FMI 10Open circuit. Repair: Check the cable connections between the control unit and the clutch actuator. Run a diagnostic check of the clutch actuator. Switch the ignition off, switch it on again and repeat the test. If the fault persists, replace the control unit.

SITRAK

FMI 0TPS1 opening angle greater than TPS2
FMI 1TPS1 opening angle less than TPS2
FMI 2Throttle valve mechanical stop learning value drifted out of range. Scan tool code: P2108. Effect: Fan and warm-up control may misbehave.
FMI 3High voltage on TPS1
FMI 4Low voltage on TPS1
FMI 5Throttle valve actuator circuit open load. Scan tool code: P2100. Effect: Torque may be limited.
FMI 7Unable to reach higher TPS opening angle
FMI 8Long-term drift of the throttle valve closed-position learning value. Scan tool code: P2108. Effect: Fan and warm-up control may misbehave.
FMI 9Long-term drift of the throttle valve open-position learning value. Scan tool code: P2108. Effect: Fan and warm-up control may misbehave.
FMI 11Intermittent throttle valve fault. Scan tool code: P2620. Effect: Torque may be limited.
FMI 12Throttle valve governor deviation at cold start. Scan tool code: P2112. Effect: Fan and warm-up control may misbehave.
FMI 13Throttle valve feedback sensor voltage above the upper limit. Scan tool code: P2622. Effect: Torque may be limited.
FMI 14Throttle valve feedback sensor voltage below the lower limit. Scan tool code: P2621. Effect: Torque may be limited.
FMI 15Throttle valve fully-open learning value beyond the limit. Scan tool code: P2108. Effect: Fan and warm-up control may misbehave.
FMI 16Persistent negative throttle valve governor deviation. Scan tool code: P2111. Effect: Fan and warm-up control may misbehave.
FMI 17Throttle valve closed-position learning value beyond the limit. Scan tool code: P2108. Effect: Fan and warm-up control may misbehave.
FMI 18Persistent positive throttle valve governor deviation. Scan tool code: P2112. Effect: Fan and warm-up control may misbehave.
FMI 20Throttle valve position signal above the mechanical maximum. Scan tool code: P2101. Effect: Fan and warm-up control may misbehave.
FMI 21Throttle valve position signal below the mechanical minimum. Scan tool code: P2101. Effect: Fan and warm-up control may misbehave.
FMI 22Short-term drift of the throttle valve closed-position learning value. Scan tool code: P2108. Effect: Fan and warm-up control may misbehave.
FMI 23Short-term drift of the throttle valve open-position learning value. Scan tool code: P2108. Effect: Fan and warm-up control may misbehave.
FMI 24Over-temperature of the throttle valve powerstage. Scan tool code: P2101.
FMI 31TPS1/2 tracking voltage out of range

YaMZ

FMI 2Intake air throttle valve seized (valve seized); or: Intake air throttle position sensor fault, implausible signal (blink code 6-7-7). Blink code: 6-7-4. Repair: Check the condition of the intake air throttle actuator.
FMI 3Intake air throttle control valve fault (control signal above allowable limit); or: Intake air throttle position sensor fault (blink code 6-7-6); or: Intake air throttle actuator fault (blink code 6-7-8). Blink code: 6-7-3. Repair: Check the condition and connection of the intake air throttle control valve.
FMI 4Intake air throttle control valve fault (control signal below allowable limit); or: Intake air throttle position sensor fault (blink code 6-7-6); or: Intake air throttle actuator fault (blink code 6-7-8). Blink code: 6-7-3. Repair: Check the condition and connection of the intake air throttle control valve.
FMI 12Intake air throttle valve actuator fault (General fault). Blink code: 6-7-8. Repair: Check the intake throttle actuator and its wiring.
FMI 20Intake air throttle valve position control fault, sustained deviation (Adaptation coefficient deviated significantly from initial value). Blink code: 6-7-5. Repair: Check the intake throttle actuator.
FMI 21Intake air throttle valve position control fault, deviation from target (Adaptation coefficient deviated significantly over the drive cycle). Blink code: 6-7-9. Repair: Check the intake throttle actuator.

ZF AS-Tronic gearbox

FMI is per the block maker's own table, not the J1939 table below.

FMI 10Fault on valve Y15 output circuit (clutch slow engagement).

What FMI means in J1939

SPN 51 FMI 051/0 · 51.0data above normal range (most severe level)
SPN 51 FMI 151/1 · 51.1data below normal range (most severe level)
SPN 51 FMI 251/2 · 51.2data erratic or intermittent
SPN 51 FMI 351/3 · 51.3voltage above normal or shorted high
SPN 51 FMI 451/4 · 51.4voltage below normal or shorted to ground
SPN 51 FMI 551/5 · 51.5current below normal or open circuit
SPN 51 FMI 651/6 · 51.6current above normal or grounded circuit
SPN 51 FMI 751/7 · 51.7mechanical system not responding
SPN 51 FMI 851/8 · 51.8abnormal frequency or pulse width
SPN 51 FMI 951/9 · 51.9abnormal update rate
SPN 51 FMI 1051/10 · 51.10abnormal rate of change
SPN 51 FMI 1151/11 · 51.11root cause not known
SPN 51 FMI 1251/12 · 51.12faulty device or component
SPN 51 FMI 1351/13 · 51.13calibration out of range
SPN 51 FMI 1451/14 · 51.14manufacturer-specific instructions
SPN 51 FMI 1551/15 · 51.15above normal — low severity
SPN 51 FMI 1651/16 · 51.16above normal — moderate severity
SPN 51 FMI 1751/17 · 51.17below normal — low severity
SPN 51 FMI 1851/18 · 51.18below normal — moderate severity
SPN 51 FMI 2051/20 · 51.20data drifted high
SPN 51 FMI 2151/21 · 51.21data drifted low
SPN 51 FMI 3151/31 · 51.31condition exists (see manufacturer description)

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

Time you have: Enough to reach the yard

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

Makes with this code

MAZYaMZSITRAKRenault TrucksCummins ISBKAMAZ-5490Meritor (ex-WABCO)

Related problems

Scanner threw several codes at once? Paste the whole list — we will show which one is the root cause and which ones simply followed.