Error Code P1126 is defined as Throttle Control Motor Relay Circuit Open. This means the voltage power source for throttle control is excessively low, which could be a case of leak in the seal or the thermostat is stuck open.

This engine code is a generic trouble code, meaning it applies to all vehicles equipped with OBD-II system, or vehicles made since 1996 up to present. Specifications on the definition, troubleshooting and repairs of course, vary from one make and/or model to another.


The power supply for the throttle control provides the PCM (powertrain control module, also known as ECM or engine control module) through throttle control motor relay. The latter is turned On/Off by the former; when the ignition is switched off, the PCM will send a signal to the throttle control motor relay and a battery voltage is provided to the PCM. When the ignition is switched off, the PCM will send an off signal to the throttle control motor relay and battery voltage will NOT be provided by the PCM.

Error Code P1126 is determined when there is an excessively low voltage power from the throttle control motor detected by the PCM. Or the engine coolant temperature doesn’t reach the temperature specified by the manufacturer even though the engine has run long enough.

Common Symptoms

Check engine light is on and error code P1126 is set.

Possible Causes

  • Faulty throttle control motor relay
  • Open or shorted throttle control motor relay harness
  • Poor electrical connections in throttle control motor relay harness
  • Low engine coolant level
  • Stuck open or faulty thermostat
  • Leakage in the seal part of thermostat
  • Faulty engine coolant temperature sensor

How to Check

The first thing technicians usually do to diagnose for this error code is to measure the resistance between pins 3 and 4 of the magnetic clutch circuit. The resistance must be somewhere between 4.2 and 5.2 ohms. If it is either open or shorted, then the problem is confirmed.

How to Fix

  • Fixing coolant leaks
  • If no coolant leaks, replacing thermostat will usually fix the problem