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P0804 is a diagnostic trouble code (DTC) for "Upshift (Skip Shift) Lamp Control Circuit Malfunction". This can happen for multiple reasons and a mechanic needs to diagnose the specific cause for this code to be triggered in your situation. Our certified mobile mechanics can come to your home or office to perform the Check Engine Light diagnostic. Once we are able to diagnose the problem, you will be provided with an upfront quote for the recommended fix and receive $20.00 off as a credit towards the repair. All our repairs are backed by our 12-month / 12,000-mile warranty.
Your vehicle’s powertrain control module (PCM) has detected a malfunction in the one through four skip shift solenoid lamp control circuit. The one through four skip shift solenoid control circuit is also known as the upshift control circuit because it prevents you from shifting through second and third gears. This is to improve fuel efficiency by eliminating the higher RPM levels that come with shifting through lower gears. The P0804 code lets the driver know that the one through four skip shift system has been turned on. Please note that this pertains only to vehicles with manual transmissions. If you have an automatic transmission and this code appears, you should suspect that you have a defective PCM or that there has been a programming error.
If your PCM recognizes an abnormal voltage or resistance reading from the skip shift lamp control circuit, the P0804 code will be stored and you will see that a Check Engine or Service light will come on to let you know that something has malfunctioned. But beware that on some vehicles a lamp will not illuminate to let you know something is wrong. In these cases, you will have to trust your vehicle’s symptoms.
According to most technicians, the most common causes of the P0804 code are a defective skip shift indicator lamp bulb, skip shift solenoid, skip shift relay, fuse, or actuator. Some other causes could be open, shorted, or corroded wiring or electrical connectors. If you have installed an aftermarket skip shift, then you may also see the P0804 code appear.
Some symptoms of the P0804 code include a skip shift indicator lamp that is not coming on, a lit Service Engine Soon lamp, and a stored code.
You will need a scanner (or code reader) and a digital volt/ohmmeter to diagnose this problem. The skip shift solenoid is usually found mounted in the transmission housing near the shifter assembly. Begin with a careful visual inspection of all wiring and connectors. Make sure to repair or replace any disconnected, damaged, shorted, or corroded connectors, wiring, and other components. After the codes are cleared, start the vehicle and see if the code reappears. If the code appears immediately, using the digital volt/ohmmeter check your battery voltage on the input side of the one through four skip shift solenoid. If there is no voltage on the input side of the solenoid, disconnect any related control modules and test for continuity between the system relays and fuses along with the input circuit at the one through four skip shift solenoid. If voltage is present on the input side of the one through four skip shift solenoid, use the scanner to command the solenoid “ON” and test for voltage on the output side. If you see voltage on the input side but nothing is happening on the output side, you might have a faulty or misadjusted one through four skip shift solenoid.
A large problem with the P0804 code is that your Service Engine Soon lamp might not come on, making it difficult to know that a malfunction has occurred.
Although a P0804 is not as serious as some other codes, your vehicle should still be examined as soon as you notice the malfunction. If ignored for too long, this issue could damage other components in your vehicle.
You may need to replace or adjust your solenoid and then retest the system afterward to ensure a successful repair. If there is voltage present on the output circuit of the one through four skip shift solenoid, you should check your circuit continuity and resistance between the switch and the PCM. In extreme cases, you will have a defective PCM, but this is rare.
Like many trouble codes, a P0804 can become complicated if you do not have the proper diagnostic equipment. Therefore, if you are doing this yourself, it is important to do the proper amount of research before you begin to work. Also, do not hesitate to contact a professional technician to ask questions about the best way to solve your issue. It is always better to consult an expert when you are unsure of what you are doing.
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