P0116 on the LB7 Duramax is an engine coolant temperature (ECT) sensor range or performance code. The ECT sensor tells the ECM how hot the engine is, and that value drives cold-start fueling, glow plug operation, fan control, and fuel economy strategy. P0116 sets when the coolant temperature reading is present but does not make sense compared to other data, such as intake air temperature at startup.
On the LB7 the coolant temperature reading touches almost every part of engine operation, from how long the glow plugs stay on to when the cooling fan engages and how the ECM trims fueling for economy. A sensor or thermostat that reports the wrong temperature quietly degrades all of those functions at once. Because P0116 is a rationality code rather than a hard failure, it often points to a gradual problem such as a lazy sensor or a thermostat that is slowly wearing out, which makes early diagnosis worthwhile before it affects mileage and drivability.
What P0116 means
Unlike a hard electrical fault, P0116 is a rationality code. The ECM compares the ECT reading to expected behavior, for example how quickly coolant warms after a cold start or how closely it matches IAT after an overnight soak. If the sensor reads implausibly, warms too slowly, or is stuck, the ECM flags P0116.
Common causes
- Failing ECT sensor that reads slow or skewed
- Stuck-open thermostat causing slow, low warm-up
- Low coolant level or air in the cooling system
- Corroded connector or poor ground at the sensor
- Intermittent wiring fault in the ECT circuit
Symptoms
- Check engine light
- Poor cold-start performance or extended warm-up
- Reduced fuel economy
- Gauge reading that seems off or slow to rise
- Cooling fan running more than expected
How to diagnose it
Watch ECT and IAT together with a scan tool after the engine has sat overnight; they should read within a few degrees of each other. A large split points to a skewed ECT sensor. Then monitor how coolant temperature climbs during warm-up. Slow, incomplete warm-up suggests a stuck-open thermostat rather than the sensor. Inspect the connector and ground, and check coolant level and condition. Compare the sensor resistance to a temperature chart if needed.
How to fix it
Top off and bleed the cooling system if it is low, and replace a stuck-open thermostat if warm-up is slow. If the sensor itself reads skewed against IAT at a cold soak, replace the ECT sensor and repair any corroded connector or ground. Clear the code, confirm ECT and IAT agree at the next cold start, and verify normal warm-up on a road test. For an ECT sensor or thermostat for your LB7, contact sales@coredieselparts.com.
Frequently asked questions
Can a bad thermostat cause P0116?
Yes. A thermostat stuck open lets the engine warm slowly and run cool, which the ECM sees as an implausible coolant temperature and flags as P0116.
Does P0116 affect fuel economy?
It can. If the ECM thinks the engine is colder than it is, it may enrich fueling and delay closed-loop strategy, hurting mileage until the fault is fixed.
How do I know if it is the sensor or the thermostat?
Compare warm-up behavior and the ECT-versus-IAT split at a cold start. A slow warm-up points to the thermostat, while a skewed reading at soak points to the sensor.
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