Throttle Position Sensor Voltage Too Low
The KOHLER-COMMAND-P0122 (Kohler Command) diesel fault code means: Throttle Position Sensor Voltage Too Low. This is a serious severity code.
- Keep driving?
- Yes, but fix soon
- DIY difficulty
- moderate
- Estimated cost
- $25-$120 DIY (TPS sensor replacement plus connector repair kit). $150-$350 at a dealer shop including diagnosis labor. ECM replacement if required can run $400-$600 in parts alone.
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Common Symptoms
- Engine starts but RPM stays low and does not respond normally to load
- Mower surges or hunts for RPM under cutting load
- EFI fault indicator light on the equipment dash flashes or stays on
- Engine enters limp-home mode and will not reach full governed speed
- PTO engagement causes engine to bog severely or nearly stall
- Blink code sequence on the ECU diagnostic LED indicates a TPS fault
- Engine may shut down unexpectedly under heavy grass conditions
Probable Causes (Ranked by Likelihood)
- TPS signal wire shorted to ground or chafed against frame/engine block Very Likely
- TPS connector corroded, backed-out, or loose at the throttle body Very Likely
- Failed throttle position sensor outputting voltage below the 0.5V minimum threshold Likely
- 5V reference wire from the ECM open or shorted, dropping signal voltage below range Likely
- Water intrusion into the TPS connector or wiring harness near the air cleaner Possible
- ECM internal fault causing incorrect reference voltage output to the TPS Less Likely
Step-by-Step Diagnostic Procedure
Start by pulling the TPS connector at the throttle body. Inspect every terminal for green corrosion, pushed-back pins, or cracked plastic. A dirty or loose connector causes most P0122 faults on these engines. Clean with electrical contact cleaner and reconnect firmly before doing anything else.
With the key ON and engine OFF, use a multimeter set to DC volts. Probe the 5V reference wire (typically the red or brown wire in the TPS connector) against engine ground. You should read 4.8 to 5.2V. A reading below 4.5V points to a wiring or ECM reference problem, not a bad sensor.
Without disconnecting the connector, back-probe the TPS signal wire (typically green or yellow) against engine ground with the key ON. The signal should read between 0.5V and 1.0V at idle/closed throttle and rise smoothly toward 4.5V as the throttle plate opens. A reading below 0.3V confirms the code condition.
Inspect the entire TPS wiring harness from the throttle body back to the ECM. Look for spots where the harness contacts the exhaust, engine block, or frame. A single rubbed-through spot that grounds the signal wire will set P0122 every time. Repair any chafe with adhesive-lined heat-shrink tubing.
Measure resistance across the TPS sensor itself: disconnect the connector, then probe the two outer terminals. Resistance should be approximately 1,000 to 2,000 ohms depending on sensor make. Next probe the center (signal) terminal to one outer terminal and slowly rotate the throttle shaft by hand. Resistance should rise or fall smoothly with no dead spots or sudden jumps. Any spike or open reading means the sensor is mechanically worn.
If wiring and sensor resistance both check out, reconnect the TPS and check for voltage drop in the ground circuit. Probe from the TPS ground pin directly to battery negative. Voltage drop should be less than 0.1V with key ON. More than 0.2V means a bad ground path that is pulling the signal reference down and causing the low voltage code.
If all wiring and sensor tests pass and the code returns, the ECM may not be supplying correct 5V reference. At this point you need the Kohler PCM diagnostic tool or a dealer with Kohler EFI software to read live TPS voltage data and check ECM output parameters. Do not replace the ECM without confirming the reference voltage failure first.
Frequently Asked Questions
What does Kohler code P0122 mean on my commercial mower?
P0122 means the ECM is reading a voltage signal from the throttle position sensor that is lower than it should be. The normal closed-throttle signal is around 0.5 to 1.0V DC. When it drops below roughly 0.3V, the ECM flags P0122 because it cannot accurately calculate how much throttle you are asking for. The engine typically falls into a safe, limited RPM mode to protect itself.
Can I keep mowing with a P0122 fault active?
You can often keep the engine running, but performance will be reduced. The engine will not reach full governed RPM, and engaging the PTO for blades may cause heavy bogging or a stall. Running in this condition through heavy grass risks overheating the engine and stressing the EFI system. Finish the current pass if you must, but diagnose and repair it before the next full job.
How much does it cost to fix a P0122 on a Kohler EFI engine?
If it is just a dirty or loose connector, the fix is free. A new TPS sensor runs $25 to $75 in the aftermarket or $80 to $120 through a Kohler dealer. If a wiring harness section needs repair, add $20 to $50 in materials for DIY. A dealer shop will typically charge $150 to $350 total including diagnosis time. ECM replacement, which is rarely needed, adds $400 or more in parts.
Will a P0122 fault prevent my mower from starting at all?
Usually no. The engine will still start and run in a reduced capacity limp-home mode. On some severe cases where the signal wire is completely open or shorted hard to ground, the engine may crank and start but immediately hunt and stall. It is uncommon for P0122 alone to cause a complete no-start, but pairing it with a low battery or other sensor fault can make starting unreliable.