P0118 Code - Engine Coolant Temperature Circuit High

[007J]

Hey guys, so I'm scratching my head here a little bit. I've replaced the thermostat, the fluid, the gray 2 pin engine coolant temperature sensor, and the 2 pin connector itself. I properly burped it and refilled it after I replaced the sensor and then I disconnected both battery terminals for an hour but the code came back on the second start up. I'm considering doing a more thorough flush again with new fluid. I did the sensor last about 1k miles later and the fluid already had a bit of debris and discoloration. My dash reads normally without any jump in the gauge at all, and I thought the 2 pin sensor meant it only sent the signal to the ECM and then to the dash, instead of the 4 pin sensor sending to both the dash and the ECM at the same time. That makes me think its not a broken wire sense my dash is reading okay. Any corrections on my logic or suggestions here?

[007J]

Has anyone else had trouble getting rid of this code or is it just me?

[Cyrik]

Hey guys, so I'm scratching my head here a little bit. I've replaced the thermostat, the fluid, the gray 2 pin engine coolant temperature sensor, and the 2 pin connector itself. I properly burped it and refilled it after I replaced the sensor and then I disconnected both battery terminals for an hour but the code came back on the second start up. I'm considering doing a more thorough flush again with new fluid. I did the sensor last about 1k miles later and the fluid already had a bit of debris and discoloration. My dash reads normally without any jump in the gauge at all, and I thought the 2 pin sensor meant it only sent the signal to the ECM and then to the dash, instead of the 4 pin sensor sending to both the dash and the ECM at the same time. That makes me think its not a broken wire sense my dash is reading okay. Any corrections on my logic or suggestions here?

Never had experience with this personally but I thought I'd shed a tad bit of light on this.

The coolant temp sensor should function like most temp sensors; it outputs a certain resistance level based on temperature. If the coolant temp is low, the resistance the sensor is outputting to the ECU should be high, and when coolant temp is high, resistance should be low. Did you use an OEM sensor for replacement? A cheapo temp sensor could absolutely throw off readings and cause a code if the ECU is measuring resistance that's out of the range it's expecting. Hope some of this info helps, also double check your wiring, of course :) (probably not disconnected or shorted otherwise your code would probably be listed as such, but never hurts to double check)

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[Cyrik]

Also if you have VCDS you could log and see what the actual value that sensor is giving. Don't quote me on this but you should see around 2000 ohms when cold.

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[noyesclt]

Audi A4 B7 - P0118 G65 Sensor - SOLVED

As Summer heats up having my son’s AC not working is a big deal. I purchased the car used and it has never worked, throwing a P0118 error. I’m a fairly technical guy (former NASA engineer) but not an auto mechanic. I thought I’d walk everyone through how I solved this very complex puzzle so that someone can save themselves the 100 hours I’ve spent on it. My A4 was used with extensive work (of unknown quality). Given the number of broken connectors I don't think an Audi dealer was doing all of the repairs.

I have lots of tools including signal testers and wave form generators. Also have VCDS (highly recommended $200) and the Bentley manual. Some of the pages are in links above for this issue.

Key Lessons
1) DO NOT buy parts until you diagnose
2) Audi control systems are complex. The computer controls the compressor function, if it does not obtain a proper signal it will not turn on the compressor.
3) Purchase a Bentley manual
4) Start with the Ross Tech HVAC season Diagnosis Post before you do anything
5) Go ahead and remove the bumper. It takes 15-20 min.. Its not really that bad.
6) Buy a Vehicle super probe to read resistance, voltage and waveform. (mine is here $70 on Amazon)
7) Drive to your local parts store and read the refrigerant pressures on both high and low side.

Troubleshooting and diagnosis

A. P0118 preventing AC Compressor from engaging. Link to Ross Tech overview of P0118 is above with detail below. 00819 - High Pressure Sensor (G65): Open or Short to Ground

Possible Causes
1) Wiring/Connectors from/to High Pressure Sensor (G65) faulty
2) High Pressure Sensor (G65) faulty
Possible Solutions
1) Check Wiring/Connectors from/to High Pressure Sensor (G65)
2) Check High Pressure Sensor (G65)

B. VagCom reading of HVAC Log blocks 1 and 2 show 0% duty cycle and 33bar (100%) high pressure signal with shut off codes 1,4 and 3. These codes can be interpreted here.

C. Removing the front bumper and disconnecting the pressure sensor I measure voltage of 3 pins. The PIN diagram is show in the Bentley manual above. For my car, Pin1 is brown wire (ground), Pin 2 is Signal Wire (Brown/Blue) and Pin 3 is power on A38 wiring harness. Using a multimeter I check the connector for proper voltage. Pin 1 is ground so checking PIN 1 Voltage to Battery+ shows battery voltage. Pin 2 is Signal and shows 9.9V, PIN 3 is power and shows battery voltage. Additionally I check Pin2 continuity to both the Station A Pillar Connector14 (T10d) in wiring diagram above. The A pillar connector is located near drivers left foot. Just take off the fuse cover and the molding should come straight out. Continuity (resistance) was confirmed.

D. Next is to check the signal wave form for the current G65 pressure switch using my Vehicle probe. I did not want to take apart the connector to the G65 switch, so I went back to the connected 14 connecter (T10/d) in the drivers footwell to touch the vehicle probe to the T10/d connector while the probe was in wave form reading. With the car running and air conditioning turned on the reading is shown below. It was pegged at 100% and 10.4 volts .
Picture of probe readings


E. Given that I had established that the G65 connector was fine and there were no wiring issues. I thought that this signal indicated I needed a new G65 sensors. After ordering and installing. The same problem persisted. I then ran across this Audi TSB for G65 (for 2009 A4 and mine is 2006.. But G65 sensor is same). It this TSB they provide overview of wave form and process to diagnose.


F. The waveform above is square wave 50Hz, 12 V. From #4 above, PIN 2 (signal wire) unplugged from the G65 sensor was reading 9.9 V (car on).This great youtube video on checking signal. Gave me a great idea. Why not artificially give the ECU the proper signal just to confirm everything else works.

G. I purchased a $30 signal generator and did a quick calibration for 50Hz Square wave and 10V. Then injected the generated signal into Pin 2 while grounding to Pin 1. Everything worked.. Air conditioning started. While running I recharged the air conditioning (at added quite a bit).

H. Then I turned car off, reconnected the connector to G65, and ran A/C again. Hooking up VCDS showed a fully operational G65 with normal readings. Somehow the very low pressure in the AC (now recharchaged) or a the artificial 50hz signal allowed everything to work again. It looked like the previous owner struggled with this problem. I see a new A/C pump and G65 also looked new. My guess is that some how the ECU needed a reset. Note that prior to this fix, I did measure A/C pressure on both high/low side. They were low but within limits and adding refrigerant to low side did not resolve the G65 P0118.

This was the hardest troubleshooting project I’ve ever had. My Audi dealer wanted $2k from me and said I needed a new master climate control panel. I spent about $50.. It made no sense at all. Thought I would share.

Great Audizine and VCDS links you should review on this topic
- https://forums.ross-tech.com/index.php?threads/633/
- http://wiki.ross-tech.com/wiki/index.php/00819
- https://forums.ross-tech.com/index.php?threads/10880/
- https://forums.ross-tech.com/index.php?threads/12600/
- https://www.audizine.com/forum/showt...er-replacement
- https://forums.ross-tech.com/index.php?threads/633/

[007J]

Nice work and excellent write up! I should have updated this when I finally solved the problem myself. I had the same code, but it ended up being the wiring for me. I originally started with changing the thermostat. In the midst of doing that, I broke the connector to the ECT sensor and realized I had been sold the wrong replacement sensor ( a green 4 prong sensor instead of the grey 2 prong). Replaced the sensor and then also replace the connector. Still the same code. I then got a multimeter to test the current like Cyrik advised and saw I was getting adequate power (4.65v iirc). Upon closer inspection with a very bright light down into the connector I saw the female wires coming from the harness weren't entirely seated in the connector. When the plastic purple wire retaining clip is pushed flush inside of the connector, Audi wiring will audibly snap into place in the connector if you push it with a pick from the other side. I recently re-did the job for a local friend who has a 2007 b7 2.0t (his connector to the ECT sensor came undone and got melted by the cat, same open circuit P0118 code) and it's easier to pre-seat the wires and then slide the plastic purple wire clip onto the inner connector. In all the countless video guide hours I've watched, never heard that tip for redoing Audi wiring connectors.

[Theiceman]

The ecu does not need to be " reset" it just looks for all conditions to be set or it won't turn on compressor.
I guess anytbing can hang it, but taking power away and sometimes you touch battery leads together to discharge any caps and good to go.

200 hours troubleshooting seems extreme.

Glad it's working.


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