Is it really the engine computer? What to check before you replace it

Engine computers do fail, but a lot of no-starts and stalls that look like a bad computer turn out to be something cheaper. These are the checks we walk customers through on Chrysler, Dodge, Jeep and Ram vehicles before anyone buys a part.

Read the codes before you disconnect the battery

Disconnecting the battery can erase stored trouble codes, and those codes are the best clue you have. Read them first with a scan tool. Many auto parts stores will read them for free.

Does it crank?

“Won’t start” covers two different problems, so settle this first.

• It doesn’t crank at all (the engine doesn’t turn over). Check the battery and its cables, the starter and starter relay, and the park/neutral switch. On some newer models the engine computer controls the starter relay, so it isn’t ruled out completely, but check these first.
• It cranks but won’t fire. Turn the key on and listen for the fuel pump running for a second or two, and check for spark. A failed crankshaft or camshaft position sensor is a common cause, and it costs far less than a computer.

It starts, then dies after two seconds

That pattern often points to the immobilizer. With the key on, a flashing security light on the dash means it didn’t recognize the key, so try your other key. (A security light that flashes with the key off and the doors locked is just the alarm arming, which is normal.)

A failed fuel pump or fuel pump relay can look the same, though, because the engine runs for a couple of seconds on the fuel pressure left in the line. Check for spark while cranking after it dies. No spark points toward the immobilizer or the automatic shutdown (ASD) relay circuit. Spark present points toward fuel delivery.

Several things died at once

If the radio, gauges and fuel pump all quit together, the cause is usually something they share: the ignition switch or its connector, a main fuse, or a power feed. On 1998 to 2001 Dakota and Durango models, a failing ignition switch is a well-known cause.

Dead gauges or “no bus” on older trucks

On many 1996 to 2004 trucks, the instrument cluster gets information such as the odometer reading from the engine computer over a data line. A flashing odometer, dead gauges or “no bus” on the display means the cluster isn’t hearing the computer. That can be a bad computer, but it can also be a blown fuse feeding the computer, a bad ground or a damaged wire in the data line.

Look at the plugs and grounds

With the battery disconnected, unplug the computer’s connectors and look for green corrosion, pushed-back pins or water. A loose or corroded connection can make a good computer act like a bad one.

It can fool you after a swap, too. Replacing the computer reseats every plug, so a vehicle with a bad connection may run fine for a day or a few weeks and then fail again.

When it really is the computer

If these checks come up clean and the codes or a shop point to the computer, replace it with one that matches your part number or VIN. Ours ship programmed to your VIN with the immobilizer memory reset, so there’s no dealer visit. Find yours on our home page.

Not sure yet? Call or text 812-510-3163 with your VIN and what the vehicle is doing, and we’ll help you check the simple things first.