P011A: Engine Coolant Temperature Sensor 1/2 Correlation

Some engines have two coolant temperature sensors that check each other. P011A is set when both are electrically fine but their readings differ more than a calibrated amount under conditions where they should agree. Mazda documents the code for the 2013–2016 CX-5 and 2014–2016 Mazda6: sensor 2 sits in the heater hose, and a clogged heater core keeps it noticeably colder, so P011A often comes with weak cabin heat although both sensors are good. Mazda's check is to compare both sensors with ambient temperature after an overnight soak and then the temperature difference across the heater hoses. A block heater used below 0 °C can also trigger it.

Symptoms

• Check engine light on
• Weak cabin heating (typical on Mazda)
• Otherwise often no driving symptoms

Most likely causes

• Clogged heater core or restricted coolant flow at sensor 2
• One of the two sensors drifting
• Air in the cooling system or low coolant level
• Block heater use in frost
• Contact resistance at one sensor

What to check, in order

Start with the simplest. You can tick off each step.

1. With the engine cold, check the coolant level; never do this when the system is hot
2. Compare both signals at the same time in live data and check any recent installation work.
3. Check the actual temperature in live data
4. Make sure the fan comes on when it should
5. Check the thermostat and connector
6. Measure the sensor and replace it if its readings do not make sense

Good to know

• Repair difficulty: medium
• It affects emissions.

What each character of P011A means

• P: Powertrain: engine and transmission
• 0: Generic code: same on every make
• 1: Air and fuel
• 1A: Specific fault in that system

Frequently asked questions

Can I keep driving with code P011A?

Watch the temperature gauge. If it climbs or a warning appears, stop the car in a safe place and turn off the engine: overheating can cause serious damage.

What causes code P011A?

The most common causes are: clogged heater core or restricted coolant flow at sensor 2; one of the two sensors drifting; air in the cooling system or low coolant level.

Related codes

• P0116Engine Coolant Temperature Sensor 1 Circuit Range/Performance
• P2183Engine Coolant Temperature Sensor 2 Circuit Range/Performance
• P011BEngine Coolant Temperature/Intake Air Temperature Correlation
• P011EEngine Coolant Temperature 1 / Ambient Air Temperature Correlation
• P011FEngine Coolant Temperature 2 / Ambient Air Temperature Correlation
• P0128Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)

Sources

• law.cornell.edu: 13 CCR 1968.2
• static.nhtsa.gov: MC 10168072 0001
• hella.com: test coolant temperature sensor

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.