Check Engine Light and Shifting Problems: What It Means & What To Do

Sun West Transmissions — Glendale, AZ. Free estimates, honest diagnosis and a 3-year / 40,000-mile warranty on qualifying rebuilds and replacements.

When the check engine light comes on at the same time your vehicle starts shifting strangely, the two are very likely connected. A check engine light and shifting problems together tell you that the vehicle’s computers have detected a fault and stored a code that explains at least part of what is going on. That is useful information, as long as it is read and interpreted correctly.

At Sun West Transmissions in Glendale, we see this combination every day. Here is what it means and how to approach it.

Why the Check Engine Light Comes On for Transmission Problems

On vehicles built since the mid-1990s, the onboard diagnostic system (OBD-II) monitors emissions-related components, and the transmission is part of that system. A transmission fault can affect engine load, speed and emissions, so many transmission problems turn on the same check engine light used for engine faults. Some vehicles also have a separate transmission warning light or message. See transmission warning light.

The engine computer and transmission computer share information constantly. In many vehicles they are separate modules; in others they are combined. Either way, a sensor problem on the engine side can cause a shifting problem, and a transmission fault can trigger a code in the engine computer.

Common Codes Behind Shifting Problems

• P0700 – a general flag that the transmission computer has its own stored fault.
• P0705 – transmission range sensor circuit.
• P0715 – input/turbine speed sensor circuit.
• P0720 – output speed sensor circuit.
• P0730 – incorrect gear ratio, often from slipping.
• Shift solenoid, pressure control solenoid and torque converter clutch codes, which vary by manufacturer
• Engine codes like misfires or throttle position sensor faults that affect shift quality

Typical Symptoms That Come With the Light

• Stuck in one gear or limp mode
• Harsh, late or erratic shifts
• Slipping or RPM flare
• No overdrive or no torque converter lockup, which can lower fuel economy
• Speedometer problems, if a speed sensor is involved

What You Can Safely Do

1. Note whether the light is steady or flashing. A flashing light usually means a severe engine misfire; reduce speed and get it checked right away.
2. Write down what the vehicle is doing and when it started.
3. Check transmission fluid level if you have a dipstick.
4. Have the codes read. Many parts stores read basic engine codes, but they often cannot access transmission-specific codes or live data.

Avoid clearing the codes before a technician sees them. Clearing erases freeze-frame data, which shows the conditions when the fault happened, and it can hide intermittent problems.

Why a Code Is Not a Diagnosis

A trouble code tells you what the computer saw, not necessarily what failed. A speed sensor code can be caused by the sensor, its wiring, a connector, a damaged reluctor or even the computer. A gear ratio code can be caused by worn clutches, low fluid or a hydraulic problem. Replacing the part named in the code description without testing is a common way to spend money and still have the problem.

At our Glendale shop we use a professional scan tool to read all modules, review freeze-frame and live data, check for manufacturer software updates and service information, test circuits, and road test the vehicle. Then we explain what we found and what it will take to fix it. Learn more about our computer transmission diagnostics.

Electrical Repairs vs Internal Repairs

Many check-engine-light shifting complaints are electrical: a sensor, a solenoid, wiring, a connector or software. Those can often be fixed without removing the transmission. Others are mechanical, where the code is the computer reporting that the transmission is slipping. Those usually need internal repair or a rebuild. Knowing which kind you have is the whole point of diagnosis.

When to Stop Driving

Stop driving if the light is flashing, if the transmission is slipping badly, if you smell burning fluid, or if the vehicle will not keep up with traffic. If the light is steady and the vehicle is in limp mode, a short, careful drive to the shop is usually possible. Avoid freeways and towing.

Check Engine Light and Shifting Problems FAQ

Can I pass emissions with this light on?

In Arizona’s emissions program areas, an illuminated check engine light typically causes a test failure. The fault needs to be repaired.

Will a parts store code reading tell me what is wrong?

It gives a starting point. Transmission-specific codes and live data usually need a professional scan tool and testing.

How much will it cost to fix?

That depends on the code, the cause and the vehicle. Call (623) 473-9800 for a free estimate.