QuestionQUESTION: My experience was the same as the previous individual in that the car doesn't accelerate correctly and the sometimes slips and the check engine light is on. It seems to rev at 5000 rpms when doing 60-70 when it should be lower. The codes were P0765, P0755, and P0155. The information seems to relate to a solenoid but what is your diagnosis?
ANSWER: This is an odd combination of codes. I'm surprised they're exactly the same as a different questioner.
P0155 indicates a problem with the oxygen sensor heater circuit for the front bank upstream oxygen sensor.
P0755 indicates an electrical issue with the underdrive solenoid circuit.
P0765 indicates an electrical issue with the overdrive solenoid circuit.
The reason your car doesn't accelerate properly and has higher than normal rpm at highway speeds is that when the transmission control module (TCM) detects either solenoid issue, it requests the engine control module (ECM) to turn on the check engine lamp and puts the transmission in third gear. The transmission will stay in third gear until the ignition is cycled off and back on.
P0155 is almost always caused by a problem with the oxygen sensor itself.
P0755 and P0765 may be caused by a problem in the transmission wiring harness. Hyundai has a very good technical service bulletin (TSB) regarding the diagnosis of these codes. In this case, you may wish to replace the solenoids in question and the transmission harness. I encourage you to register for a free account at www.hmaservice.com and peruse TSB 09-AT-018.
The reason I find this combination odd is that there are two seemingly unrelated problems. It's rare for a car to have two problems at the same time unless one has gone uncorrected. It's even less likely that two vehicles will have the same two problems occurring at the same time.
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QUESTION: Perhaps its odd because I mistyped. Good catch the wrong code was P0155- it should have been P0115. The other person explained the same scenario be provided no codes. So how would that impact your diagnosis? You obviously are an Expert.Thanks!
AnswerI was thinking the code combination was odd because of the rarity of all those codes coming up together on two different cars. Since you were referring to the symptoms and not the codes, then this isn't so rare.
P0115 indicates a problem with the coolant temperature readings. This is most frequently caused by the coolant temperature sensor, but I've also seen it caused by a stuck open or closed thermostat or any other cause of the engine overheating. Additionally, even though I've never seen it, this code could also be caused by a problem in the temperature sensor circuit wiring or in the ECM. A scan tool/code reader capable of reading freeze frame data will be valuable here, as it will tell you the coolant temperature at the time the code set. If the reading is outside the parameters of what could be possibly correct, you stand a near 100% chance replacing the sensor will be the remedy for this code. If the data indicate overheating, you'll need to audit the cooling system. If it indicates a reasonable coolant temperature below operating temperature, you may have an issue with the thermostat being stuck open. This code, too, is unrelated to the transmission problem.
