P166C Code:

OBD-II Diagnostic Powertrain (P) Trouble Code

Possible Cause

Common causes of P166C include a faulty electronic pressure control solenoid, hydraulic blockages inside the internal transmission fluid passages, mechanical internal transmission failures, low transmission fluid, dirty or contaminated transmission fluid, a faulty PCM or transmission control module.

Common Symptoms

This type of code usually represents a malfunction in the controller area network, or "CAN"
If the resistance level does not decrease with heat and increase as it cools, replace the sensor. If everything has checked out thus far, turn the key to the "ON" position and test for the presence of a voltage (and a ground signal) at the exhaust gas temperature/pressure (or heated oxygen) sensor electrical connector
A scanner (or code reader) and a digital volt/ohmmeter will be required to successfully diagnose this code. Virtually every vehicle (with an automatic transmission) produced since the mid-eighties is equipped with some type of computer controlled automatic transmission system
If the fuel shutoff switch and all system circuitry checks out, suspect a defective PCM (or other related controller)

Similar Problems

Auxiliary Emission Controls

Air Conditioning (A/C) Clutch Feedback Circuit High Voltage

TFP Valve Position Switch-Drive Without Drive Ratio

P166C Troubleshooting

A sure sign of this is smoke or steam being emitted from underneath the hood. The ignition timing could be set incorrectly and will require adjustment. If, when looking under the hood, you notice that the drive belts appear loose, they need to be tightened or completely replaced. If there is a noticeable pool of fluid underneath the car, this could be a sign of the cylinder head gasket failing. That will be confirmed in the form of billowing smoke being blown from the exhaust system. P166C OBD2 may also be triggered by faults earlier down the line. For example, a dirty MAF sensor might be causing the car to overcompensate in its fuel-trim adjustments. As a result, oxygen sensors are likely to report fuel mixture problems.

How to turn off P166C check engine light ?

There are two ways to turn off the check engine light:

A repair technician can do so once repairs have been made.
The OBD II can automatically turn it off when it fails to detect the problem after several diagnostic cycles.