Check engine light and transmission codes

Common causes

1. A sensor or wiring faultSpeed, range and fluid temperature sensors feed the computer. A bad sensor, a damaged connector or a chafed wire sets a circuit code while the hardware is fine.
2. A shift or pressure control solenoidSolenoids select gears and set line pressure. A stuck or electrically weak one sets a P075x or P074x style code and often changes how it shifts.
3. A torque converter clutch circuit faultThe lock-up clutch inside the converter is switched by a solenoid. A fault there sets a P074x code and may cause a shudder at cruising speed.
4. A real mechanical problem the computer detectedA code like P0730, incorrect gear ratio, means the computer saw engine and output speed disagree, which can come from internal slipping.

A scanner at a parts store takes a few minutes and gives you a letter and four digits. Those digits are worth writing down exactly.

At a glance

• P means powertrain; the 07 in P07xx means transmission
• A 0 as the second character means a generic, standard code
• A code names a circuit or condition, not a failed part
• Write down every code and whether the light was steady or flashing

How to read a code, character by character

The first letter names the system: P is powertrain, which covers the engine and the transmission. The second character is 0 for a generic code that every maker uses or 1 for a maker's own (2 is also generic, and 3 is split between the two). The third character names the subsystem, and 7 means transmission. The last two digits identify the specific circuit or fault. The scheme is set by the SAE J2012 standard.

So P0715 reads as: powertrain, generic, transmission, input speed sensor. The table on this page lists the generic P07xx titles.

The five codes people look up most

P0700 says the transmission control system has stored its own fault. It is a flag, not a cause; the real code is in the transmission computer and needs a scanner that reads it.

P0715 is the input or turbine speed sensor circuit. It can come from the sensor, its wiring, or from a transmission that is not turning the way the computer expects.

P0730 is incorrect gear ratio. The computer compares engine speed to output speed in each gear, and the numbers do not match, which can mean slipping.

P0740 is the torque converter clutch circuit. It often arrives with a shudder or poor mileage on the highway.

P0750 is shift solenoid A. The gear it controls may stay stuck, or shifts may be harsh.

What a code is not

It is not a shopping list. A code for a solenoid circuit does not always mean the solenoid is bad; the wiring, the connector or the fluid can set it. It is also not a reason to replace the transmission, and clearing it does not repair anything. The light comes back when the computer runs its test again.

Steady light, flashing light

A steady light is a fault that should be looked at soon. A flashing light usually means a severe engine misfire that can damage the catalytic converter: slow down, avoid heavy loads and call the shop. If the transmission also drops into one gear, see limp mode.

Getting the codes read

Many parts stores will read a code for you with a handheld scanner. Write down every code, in order, along with any that show as pending. Codes outside P0700 to P0799, including maker-specific P1 codes and U network codes, are not in this table, so read them to the shop when you call. The transmission diagnosis page describes what happens next.

Transmission codes P0700 to P0784, in plain English

A code names the circuit or condition the computer flagged, not the part that failed.