Riding Lawn Mower Won't Start? Common Causes and What to Check First

When a riding lawn mower won't start, it doesn't always mean there's some serious mechanical problem. Sometimes the problem is bad fuel, a power take-off (PTO) that's engaged, a weak battery, a faulty fuse or one of the safety switches built into the machine.

Other times you might need to check the starter circuit, fuel system or ignition system. Riding mowers include:  zero-turn mowers, lawn tractors and riding lawn mowers. In each of these, the logic used to troubleshoot starting problems is similar, though the engine, safety switches, fuel system and starter circuit will differ between models and manufacturers.

The first piece of advice is this: before doing any testing or repair, check the owner's manual and start with some basic checks.

Before You Start: Safety First

Place the mower on level ground, put on the parking brake, turn the PTO off and take out the ignition key.

Before working on the deck, blades, belts or moving parts, turn off the engine, let all moving parts stop and (if your manufacturer recommends it) pull the spark plug wire.

Never run a gasoline engine in a poorly ventilated area, such as a garage. That could expose you to carbon monoxide. Also don't try to get around safety switches on your mower or use the mower without them.

UDC Parts expert tip: If you think a safety switch is faulty, test it correctly or replace it. Do not bypass safety switches, because this can make the mower unsafe to use.

1. Start With the Basics: Controls, Fuel, and Oil

Before proceeding to use a voltmeter or opening the engine cover, evaluate the easy solutions first. Switch the PTO OFF. Most riding mower models won't permit the engine to start with the blades or PTO in operation.

Many riding lawn tractor designs require the brake or clutch to be fully pressed or the parking brake to be engaged. Some models even demand the transmission to be switched to the parking or neutral mode.

The seat switch is one of the components of the safety system in many riding mower designs. Depending on the model of the machine, it may disable or enable starting or stopping the engine if the operator is away from the seat. Do not press the switch by hand or eliminate it.

Be sure to check the fuel level, as well as the fuel shutoff valve if there is such an option available for the mower, and the length of time the fuel has been in the fuel tank.

Expired gasoline can release its volatile elements and form deposits in your carburetor or your fuel system. If your mower has been left idle for some time, consult the manufacturer on how to best replace the old gasoline.

Check the oil level as described in the instructions. Not all riding mowers are fitted with a system that disables starting the engine if the oil level is too low. Some engines feature low-oil protection or oil-pressure monitoring, but others are not so equipped. Low oil levels should be regarded primarily as a risk to the engine.

2. Clicking, Slow Cranking, or Silence: Check the Battery

A riding mower's technical mechanisms work on a 12-volt electrical power system whereas a push mower doesn’t have to.

When you turn the ignition key and hear only one click, a series of clicking sounds, very slow cranking, or no sound at all, you should check the battery and battery connections.

A completely charged lead-acid battery should read around 12.6 to 12.8 volts in a resting state. But resting voltage doesn’t provide a complete picture of the problem. It is possible that under the load of the starter a working battery can produce much lower values than its resting voltage. Therefore, make use of the load and cranking voltage tests for further testing of the battery.

Check the positive terminal, the negative terminal, the ground cable connected to the engine or chassis, and the connection at the starter or solenoid. Even a good battery will not be able to start the engine if you can see the white or green corrosion or loose screws at battery terminals.

Several problems may appear during the winter storage of a riding mower battery. If the battery is several years old and doesn't hold a charge, test it instead of repeatedly recharging it.

UDC Parts expert tip: Clean and tighten the battery terminals and ground connection before replacing the starter. A poor connection can create the same symptom as a failed starter motor.

3. If the Starter Is Silent, Check the Safety Interlocks

The starting system of a riding mower is often more complicated than that of a push mower, according to some models.

This means that the system might include parts such as interlock switches associated with the brake or clutch, PTO, neutral or park position, and others.

But not all machines function the same way, while one machine model will not respond at all when PTO is on.

Also, the starter's response depends on how a given mower operates with respect to the control positions, fuses, visible damage as well as wiring diagram.

The ignition switch of a supposed mower must have been checked as well. It supplies the power to the starting circuit, which may have gone through several safety switches before reaching the solenoid. It is possible that a switch is faulty or that there exists a bad connection.

Don't replace the ignition switch. You should always test the circuit.

4. If the Starter Cranks but the Engine Does Not Start

If the starter is cranking the engine properly, it can be assumed that battery failure is not the issue. For a gas engine to operate, the right fuel mixture must exist, as well as the required amount of compression and ignition.

Inspect the air filter first. Remove the filter element for inspection. Paper filters should be replaced. Especially if they are extremely clogged or broken. They should have been dry or twisted.

A paper filter must not be knocked against any surfaces. Avoid using compressed air to clean it, unless allowed in the instruction manual. The foam pre-filter may only be serviced according to the manual.

Carbureted engines often have no problem starting on a cold day if the choke is working properly. Make sure that the choke linkage works properly and that the choke plates are in the right position. Two types of controls are present on the carburetor: the choke and the throttle.

If the mower runs with the choke in one position, for some seconds, and then stops, there may be issues with the fuel, the device’s carburetor, or fuel delivery system.

5. Spark Plug and Ignition Checks

If the starter is functioning properly and trying to turn the engine over but the engine will not start, checking the spark plug should be next on the list.

To check the spark plug, fire the plug up, take it out and have a look at it. Checking its state can provide some indications but will not allow us to diagnose the problem for sure. What you can find out is that the plug may show signs of being wet, covered in carbon, full of oil or damaged.  Loss of that can be explained by problems with flooding, spark strength, mixture, oil or overheating of the engine.

You can't use just one spark plug gap for every mower. Different engines like those made by Briggs & Stratton, Kohler and Kawasaki have different gaps for the plug.

The most reliable method of making sure the spark happens is doing it with the help of the inline spark tester. If spark doesn't happen even with a good plug, the ignition coil, wiring, kill switch or safety interlock, other parts of the ignition system might be the real reason for that.

In this case, it is better to use model-specific diagnostics instead of replacing parts randomly.

6. Spark Is Present, but the Engine Still Will Not Start: Fuel Delivery

If there is spark and the engine starts up but the question remains does the fuel reach the engine correctly.

There are various types of fuel systems employed in the machines. For instance, the riding lawnmower may have a gravity fed fuel system, pulse or vacuum fuel pump, electric riding mower fuel pump, carbureted or electronic fuel injection system.

So, the information regarding the removal of a hose and fuel flow does not apply to all systems.

For example, carbureted engines may face issues like a clogged fuel filter, damaged or clamped fuel line, blocked channels of carburetor, stuck float needle, or deposits formed in the carburetor while in storage. In case the carburetor has a factory drainage system. It is possible to determine the fuel level present in the bowl through the manual.

Performing a complete disassembly isn't a desirable first step if you don't have prior experience. Without proper experience, it is easy to damage the gasket, lose the jets, or improperly assemble the linkage.

There are certain steps to diagnose EFI engines including testing for fuel pressure, pump operation, relay performance, analysis of the working of sensors, wires, and fault codes of the ECU.

UDC Parts expert tip: If the engine starts for a few seconds and then dies, check fuel delivery, choke position, carburetor restriction and the safety system before moving to deeper engine diagnostics.

7. One Click, but the Starter Does Not Turn

If you hear a click sound when you turn the key, but the engine doesn't start, it means that there is a problem with your battery, cable connections, or starter. The riding mower starter solenoid is an electrical switch that sends power to the starter. Clicking the solenoid indicates that it is working, but it doesn't mean that the starter is working well.

Once you have checked the battery, wiring and confirmed that everything is in good condition, you should do further troubleshooting to test the efficacy of the starter solenoid.

Tapping the starter isn’t a recommended step in riding lawn mower troubleshooting the starter. It might only mask the problem for some time.

8. When DIY Checks Are Not Enough

If there are no problems with fuel, sparks, battery, safety system, and starter, then the problem could lie deeper.

In some small engines, if the valve clearance is not correct after a long run, it may be difficult to start the engine. The process of adjustment varies from engine to engine. Low compression may be due to issues with valves, rings, malfunctioning cylinders, or other mechanical faults. A single compression reading is often not sufficient for the diagnosis, especially in small engines with an automatic compression release.

After going through a hard blow, you may get a sheared flywheel key and subsequently, the ignition timing may change. Verification of this would require disassembling the engine.

If the engine is equipped with the EFI system, you may need diagnostic codes, wiring tests, fuel-pressure calibration, or special tools from the company.

Quick Troubleshooting Summary

How to Avoid Riding Mower Starting Problems Next Season

The ideal repair is the one you don't require at all. Your mower must be stored according to the instructions of the manual. The battery should never be left in a fully discharged state. Refer to the instructions regarding the charging or storage of the battery. Clean the mower properly and check oil and filter according to the schedule of maintenance.

Follow the guidelines for the maintenance of the utility vehicle according to the time of operations and calendar periods stated in the manual. There isn’t a single instruction to replace the spark plug every season or to replace the filter every 100 hours for all riding mowers.

What Causes a Riding Mower Not to Start?

Riding lawn mowers, lawn tractors, and zero-turn mowers tend to look alike, but their internal parts may turn out to be totally different in terms of electrical systems, engine types, and safety circuits.

Correct diagnosis isn't about revealing one “typical” fault. There is a correct sequence of steps: starting with the controls and battery, continuing with the fuel and air, working through the spark, then starter circuit, and only then moving onto the carburetor, compression, valves, or electronics.

This method gives two advantages: for one, useful machines are not taken apart in vain. For another, it becomes clear at an early stage. If the fix is very simple and if the machine is already supposed to be serviced.