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This is one of the most frequent OBD2 trouble codes. Read the full article below to know what it means, how to fix it, and what other codes may show related to it.


Diagnostic trouble code (DTC) P2181 is for Cooling System Performance


The Diagnostic trouble code (DTC) P2181 Cooling System Performance is triggered by the PCM (Powertrain Control Module) when it detects that the temperature of the coolant in the radiator is not within the acceptable range. It is either too hot or cold.

This DTC code is different from other codes that are more specific in their listing of not functioning components. This code is a general trigger that does not provide specific targets of repair or service checks. We highly recommend you check for any other related codes that may show up along with this code. If there is no other code displayed, please clear the current codes using a proper Onboard diagnostics (OBD) tool and drive the vehicle for a while until you see codes again. 

This might help you to focus your efforts, and alternatively, it could help you give your mechanic a better idea of what is the problem with your vehicle.


  • Malfunctioning or faulty engine coolant temperature sensor
  • Engine coolant level not up to the required level; due to a leak or other factors like not re-filling as required
  • Faulty thermostat (stuck in open or closed position)
  • Engine overheating due to a water pump not working, failed cooling fan, bad coolant circulation, low coolant volume not enough to cool the engine


There are a few symptoms that you can note such as:

  • You might see very high or very low readings from the engine temperate gauge, indicating some malfunction
  • If your vehicle does not have a temperature gauge and instead has a temperature warning light, your Engine Control Module (ECM) might light up the warning light to let you know that the temperature is outside the desired range on either side (hot/cool).
  • Another symptom is the indirect effect on fuel economy caused when the engine is running too cold. When your thermostat is stuck open, it will cause a continuous flow of coolant from the engine to the radiator, which will cool the entire setup. The engine will not rise to its desired operating temperatures. The ECM will then shift the air to fuel mixture to a rich blend to heat the engine, reducing the fuel efficiency. This might also cause your heater not to be able to warm up your car. 
  • The engine running cold is one possible symptom, but this might also cause your engine to heat up, which has more negative impacts. A thermostat stuck closed might stop coolant flow from the engine to the radiator, and your engine will operate at a far higher temperature than required. 
  • The malfunctioning cooling fan or pump could also be the reason for your engine heating up. This might cause your air conditioner not to work correctly and not cool your car. The engine heating causes a lot of damage to your car in comparison to your engine running cool.


P2181 should be fixed at the earliest opportunity. You need to check all possible causes but pursue them in the order of most likely to most unlikely to occur (for your specific vehicle), to pinpoint the required repairs instead of having to replace multiple expensive parts.

  • If the engine is running too cold, replacing the thermostat might be a simple fix
  • If the engine is running too hot, please ensure that you do not open the radiator cap unless the   engine is cold. Opening a radiator cap with a hot engine can cause serious injury to the person near the radiator. The coolant can hit them from the radiator, usually at very high temperatures, and can cause burns 
  • If the car is not heating up while driving, but it suddenly heats up when idling or at a stop or low speeds, it might be a radiator fan that is not functioning and causing the engine to run hot

Common mistakes

  • Opening the radiator cap while the engine is running or while the engine is still hot could cause injury to the person working near the radiator
  • First, you need to pinpoint the part that is failing before making any repairs. Different parts could cause this, and thus replacing parts that are not faulty will just be an expensive repair that does not fix the problem.

How serious is this?

The engine running cold will have less impact on your engine but will have more impact on fuel economy and cost you a lot to keep the car running in terms of fuel prices.

The engine running hot has the potential to completely break your engine as the heat can cause expansion of pistons and cylinders. Such damage cannot be repaired, and you will have to find an engine replacement

What repairs can fix the code?

repair manuals

  • If the engine is running too cold, replacing the thermostat might be a simple fix
  • If the engine is running too hot, it could be either the thermostat, the radiator fan, radiator fan clutch, the coolant levels, leaks in the flow lines, etc.
  • We recommend a thorough check of all the parts to understand the issue and replace only faulty or malfunctioning parts. You must consult a trusted mechanic to get a reasonable opinion on the code and its potential causes.

Related codes

  • OBD CODE P0118 – The OBD code P0118 is defined as the Engine Coolant Temperature Circuit – High Input malfunction

  • OBD CODE P0128 – The OBD code P0128 is defined as the Coolant Temperature Below Thermostat Regulating Temperature


We recommend that you look into this issue as soon as you can because the potential of engine damage by overheating is very high. Even mild overheating can cause a lot of expensive repairs to the engine.

While fixing this problem, you need to be careful, not to replace any parts that are not malfunctioning and rather focus on finding the faulty parts causing the extreme engine temperatures