CAT Skid Steer Won't Start: Field Diagnostic Checklist

I can not tell you how many times I have driven an hour to a jobsite only to find a dead battery or an empty fuel tank. The first thing I check on every skid steer no-start is the battery. Not just whether the lights come on—a battery can have enough surface charge to light the dash but not enough cold cranking amps to spin the starter. Load test the battery. If it reads below 10.5 volts under load, it is dead. Replace it before you do anything else.

The second obvious check is the fuel level and quality. I see a lot of skid steers in Tooele that won't start because the fuel was left untreated over a cold weekend and the paraffin has waxed up the filter. If the temperature dropped below 15 degrees and you didn't use anti-gel, your fuel filter is probably plugged solid. Crack the filter drain and see if fuel flows freely. If it comes out like jelly, you have found your problem. Drain the tank, replace the filter, and treat the fresh fuel.

The third obvious one: is the machine in neutral and the parking brake engaged? I am not kidding. I have been on calls where the operator had bumped the joystick into forward and the computer refused to let the starter engage. On CAT skid steers, the ECM monitors the transmission neutral switch. If the machine thinks it is in gear, it will not crank. Before you call a mechanic, make sure all the basic operator controls are in the start position.

CAT skid steers have a series of safety interlock switches that must all be satisfied before the starter will engage. These are the most common failure points I see in the field. The seat belt switch, the seat bar switch, and the parking brake switch all feed into the machine control module. If any one of them fails, the machine will not crank. I have replaced dozens of $30 seat belt buckles that had a contractor convinced his $80,000 skid steer needed a new engine.

The diagnostic trick is to bypass the seat switches temporarily. If the machine starts with the seat switches bypassed, you have found the problem. Replace the faulty switch and move on. I did this on a CAT 259D in Grantsville last year—the operator had spilled coffee in the seat belt buckle and it corroded the switch contacts. A quick clean and dielectric grease had his loader running again. If you have a skid steer that cranks but won't fire, that is a different problem and points to fuel or engine control issues.

Do not overlook the hydraulic lockout switch either. On newer CAT models, the hydraulic lockout solenoid must be in the 'locked' position for the starter to engage. I once spent 20 minutes on a machine in Salt Lake City before realizing the operator had accidentally left the hydraulic lockout 'on' (unlocked). The machine thought it was in a dangerous state and refused to crank. Simple fix, embarrassed mechanic.

If the engine cranks but won't start, you have a fuel or spark problem. On a diesel skid steer, that means air in the system, a plugged filter, or a failed fuel pump. Start with the fuel filter. If you have not changed it in the last 500 hours, change it now. I carry spare CAT fuel filters in my truck because they fail so often. Crack the injector lines at the cylinder head and crank the engine. If no fuel comes out, the issue is upstream—fuel pump, filter, or tank.

Air in the fuel system is the next most common cause. If someone has changed the filter or run the tank dry, air gets trapped and prevents the injectors from firing. You need to bleed the system at the filter housing and the injection pump. On CAT skid steers, the electric lift pump should run for a few seconds when you turn the key to the "on" position. If you do not hear it humming, the pump may be dead or the ECM relay may have failed. I bypassed a failed lift pump relay on a job in Heber City and had the machine running in ten minutes with a $5 jumper wire.

Finally, check for water in the fuel. Water settles at the bottom of the tank and the filter water separator. If you see water droplets in the fuel or the separator is full, drain it completely. Water in the fuel system is the fastest way to destroy injection pumps and injectors. I tell all my clients: drain your fuel water separator every 250 hours, no exceptions. It takes 30 seconds and can save you $5,000 in fuel system repairs.

If you have gone through the battery, safety switches, and fuel system and the machine still will not start, you have a deeper electrical or mechanical issue. The next step requires a diagnostic laptop with CAT ET software. The ECM stores fault codes that tell you exactly which sensor or circuit has failed. I have seen machines with a failed camshaft position sensor that would crank all day but never fire because the computer did not know when to inject fuel.

I also see a lot of bad ECM power relays on CAT skid steers. The main relay that powers the ECM can fail intermittently—the machine might start one day and not the next. Those are nearly impossible to diagnose without a wiring diagram and a multimeter. If your skid steer is acting up and you have exhausted the basic checks, call a mechanic who knows these machines. We carry CAT ET software on every truck and can typically diagnose any no-start in under an hour. If you need skid steer repair in Salt Lake City or anywhere across Utah, we can get your machine running.