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1999 Pontiac Grand Am P0117 Code: Diagnosis and Fix for Coolant Temperature Sensor Failure

Model: 99 Grand Am Fault Code: P0117 Posted: 2014-03-23 14:10

I own a 1999 Pontiac Grand Am with a 3.4L engine. After replacing the head gaskets about a month ago, the car ran perfectly initially—no check engine light, no issues during driving. However, about a week ago, I started experiencing multiple symptoms: the check engine light illuminated, both cooling fans activated after just 15 seconds of startup, the temperature gauge remained stagnant (even when warm), and the transmission would suddenly slam into gear. Using a scan tool, I only see one code: P0117—indicating an ECT (Engine Coolant Temperature) sensor circuit malfunction. When I tested the coolant temperature sensor directly with a multimeter, I observed 285°F when unplugged and 265°F when plugged in—no difference between cold or hot conditions. The expected voltage at the sensor connector is 5 volts, but my meter reads no voltage. I’ve thoroughly checked all fuses, wiring connections, and harnesses. I have not removed the upper intake manifold to inspect under-the-harness routing. My PCM is approximately one year old and has never had a previous issue. I followed a known diagnostic thread on P0117—ECT Circuit Malfunction—but still cannot identify the root cause. I’m seeking a clear, step-by-step diagnostic procedure for this specific 1999 Grand Am model to avoid further guesswork. Specifically: - What is the correct wiring configuration (2-wire: yellow and black/orange)? - How should I test for continuity or grounding between the PCM and ECT sensor? - Does a P111x code appear in addition to P0117? If so, what does it indicate? - Are there known issues with electromagnetic interference (EMI) from ignition components affecting this circuit? Additionally: When the engine sits overnight, should coolant and intake air temperatures be within 3°C (5°F)? I’ve verified that they are not—this may point to a sensor or wiring issue. I’m now considering replacing the ECT sensor due to possible paint contamination on its threads during prior work. However, after replacement, symptoms persisted. I will update with further progress once the new sensor arrives and after completing a full wiring inspection.

Related fault codes
P0117
Comments (11)
Anonymous 2014-03-23 15:10

The ECT sensor uses two wires: yellow (signal) and black/orange (ground). You report no voltage on the yellow wire—this is critical. When tested with a key-on engine-off (KOEO), the sensor reads 285°F unplugged and 265°F plugged in. This suggests a possible ground circuit between the PCM and ECT sensor. Using a digital multimeter, check for an open or short between the PCM harness and ECT sensor at the sensor end. Also confirm if your scan tool displays manufacturer-specific codes—please share any P111x codes. Was there a prior issue before this repair? Double-check for pinched or damaged wiring.

Anonymous 2014-03-23 15:20

When disconnecting the PCM, which specific pins should I test on the ECT sensor harness to verify continuity or short between the PCM and ECT sensor?

Anonymous 2014-03-23 15:24

Let’s start simple: unplug the PCM first, then unplug the ECT sensor. At the sensor end, check if the yellow wire is grounded. Since it's Sunday, I’ll wait until Monday to confirm the exact pinout—but this test will help isolate whether the grounding issue occurs at the sensor or in the harness.

Anonymous 2014-03-23 15:46

Using a test light, I observe ground on both the yellow and black/orange wires. This suggests a possible electrical short, but I’m unsure if it’s an electronic ground within the PCM or a physical grounding issue in the wiring.

Anonymous 2014-03-23 15:47

I now understand—unplug the PCM, then unplug the ECT sensor and check for a ground at the sensor end. I’ll perform this test immediately.

Anonymous 2014-03-23 16:23

With the PCM disconnected, there is no ground detected on the yellow wire. Next step: inspect the harness for intermittent faults or vibration-induced shorts.

Anonymous 2014-03-24 07:59

After gently shaking and wiggling the harness to simulate vibration, I still see no change. The only code remains P0117. I’ve tested resistance with a multimeter—no issues found. Freeze frame data shows no anomalies. I’m now planning to replace the ECT sensor. I suspect paint residue from prior work may have entered the sensor’s threads or connector area. It’s odd that everything worked fine for weeks before this. The new sensor will arrive in a few days—I’ll update with results. Any additional insights would be greatly appreciated.

Anonymous 2014-03-24 08:02

I replaced the ECT sensor—no improvement. I then traced the wiring, discovering that a section had been taped over with electrical tape. Upon removing it, I found two wires melted together—likely due to overheating or poor insulation. I cut and repaired the damaged section properly. After this fix, all symptoms resolved. Thanks for your guidance—I would not have discovered this without your help.

Anonymous 2014-03-24 09:16

No further issues reported. This case highlights the importance of inspecting taped or hidden wiring sections in older vehicles with high EMI exposure. Always verify sensor integrity and check for physical damage, especially after mechanical repairs.

Anonymous 2014-03-28 14:50

ok replaced the coolant sensor, did not change a thing, I decided to track the wiring even though I did not find anything before. I found a spot that was taped up with electrical tape and decided to untape it. Under the tape I found 2 wires melted together, not sure why someone taped it and did not fix it before. I cut the wires out and repaired them properly and this corrected my prob. Thanks for all the help, I could not have tracked this down without you.

Anonymous 2020-11-28 09:35

Don't want your junk.