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I want to share my troubleshooting I have spent for 2-3 months on an old A3. It had many issues but the most noticeable were erratic idle when cold and hot.

A3 8L 1.6 APF, automatic gearbox, no climate control

Control Module Part Number: 06A 906 033 A

Component and/or Version: SIMOS33 1,6l 2V 00AT3203

Software Coding: 00013

Work Shop Code: WSC 06435

7 Faults Found:

16555 - Fuel Trim: System Too Lean: Bank 1

P0171 - 35-10 - - - Intermittent

16845 - Fuel Level Sensor Circuit: Implausible Signal

P0461 - 35-10 - - - Intermittent

16555 - Fuel Trim: System Too Lean: Bank 1

P0171 - 35-10 - - - Intermittent

17698 - Coolant Temp Sensor at Radiator Outlet (G83): Signal too Small

P1290 - 35-10 - - - Intermittent

16554 - Fuel Trim: Malfunction: Bank 1

P0170 - 35-10 - - - Intermittent

16688 - Cylinder 4 Misfire Detected

P0304 - 35-10 - - - Intermittent

17584 - Bank1: O2S (Lambda) Correction Behind Catalyst: Control Limit Reached

P1176 - 35-00 - -

17851 - Potentiometer for EGR (G212): Signal too Low

P1443 - 35-10 - - - Intermittent

Readiness: 0000 1000

I have started with all well known fixes for this.

Firs of all,

• I welded the leaky exhaust
• changed the torn intake hose that was leaking from the oil fill neck
• changed the brake booster hose that was torn on the one-way valve (both sides)
• changed the intake actuator and hoses
• changed the G83 sensor. I could not see the fault online running VAG but obviously it was faulty
• changed the steer-asist pump pressure sensor as it was leaking
• verified the fuel pressure regulator to show 2.5 during idle and 3bar at full throttle
• cleaned the MAF
• cleaned and set the throttle body
• cleaned and verified the operation of EGR and secondary air pump and valve
• verified the output from both lambda sensors

Outcome:

• cold start has been fixed and the car runs great. I have found that the thermostat does not open at 80 °C as specified on ross-tech site. APF engine starts to open thermostat at 103 °C; If you turn the heater knob on, thermostat starts to open at 95 °C

Unfortunately, I was not able to fix the issue of random high idle. The ECU randomly increased idle from 800 RPM to 960RPM. Sometimes a blip of throttle fixed it and it settled back at 800rpm but again after some time it jumped to 960.

I have done air temp, air mass, water temp and throttle body angle value observations. I could not find anything special except that at 800rpm air mass was at 2.7 grams throttle angle at approx 2.3-2.5 ° and at 960rpm air mass was at approx 4 grams, throttle angle was at 4.1 °

This happened only when warm. The more colder it became now outside during november the less it did - this random rpm rise.

I started to check everything again and finally charging came to my mind. I must say that the car charged the battery ok but I did not trust it. I connected a simple volt meter and it showed only 13.5V at battery. Next I have seen stat on startup it has not started charging immediately but only after RPM was brought to approx 2k. This means alternator did not get any juice to the exciter winding and excited by itself. I started to inspect the L cable (blue one) from alternator to the connector just under the battery.(L DFM plug) And wuala, there it was, just sitting broken inside the connector. Fixing the L cable fixed all the issues with random RPM rise to 960. If somebody might experience a random RPM rise, it might not always be a dirty throttle body. Lesson learned!