How Long Does a Main Relay (Computer/Fuel System) Last?

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The main computer relay is responsible for supplying power to the Powertrain Control Module (PCM). The PCM is the primary computer that controls the operation of the engine, transmission, emission control system, starting system, and charging system. Other systems not directly related to emissions have some amount of PCM control.

Several symptoms are possible whenever a PCM relay starts to fail.

1. Intermittent no-crank or no-start condition

It is possible for a relay to fail intermittently. This creates a condition where the engine may crank but not start. It can also prevent the engine from cranking at all. The PCM has no power to energize the fuel injection system and the ignition system, which leads to the no-start condition. At other times, the engine will start and run normally. The most common cause of an intermittent relay failure is an open circuit inside the relay itself, usually from open soldered joints.

2. Engine not cranking or starting at all

When the PCM relay has failed completely, the engine will either not crank or not start at all. The PCM isn’t the only possible cause of a no-crank/no-start, however. Only a trained technician like those at YourMechanic would be able to determine what the actual cause is.

3. No scan tool communication with PCM.

A failed PCM relay will not allow the PCM to power up. When this happens, the PCM will not be able to communicate with any diagnostic scan tool. For a technician, not being able to communicate with the PCM increases the complexity of diagnosis.

If the relay has failed, it needs to be replaced.


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Michael

15 years of experience
518 reviews
Michael
15 years of experience
Mercedes-Benz GL450 - Brake Pads Replacement (Rear) - Atlanta, Georgia
Micheal was punctual and friendly. Work was completed within promised time. Will definitely use again.
Volvo S80 - Brake Pads Replacement (Front) - Roswell, Georgia
Michael was SUPER QUICK with doing my brake and rotors. Great communication throughout the whole process. Will always recommend

Kenneth

19 years of experience
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Kenneth
19 years of experience
Toyota Sequoia - Brake Pads Replacement (Front) - Newport Beach, California
Kenneth did a great job and very knowledgeable. He has a great attitude and I will for sure be using him again.

Chuy

13 years of experience
129 reviews
Chuy
13 years of experience
Honda Civic - Brake Pads Replacement (Front, Rear) - Cicero, Illinois
Excellent work on doing the breaks for my car and the break and steering system flush I’ll definitely recommend and take into consideration for chuy’s work whenever I need a fix on my car

Richard

16 years of experience
72 reviews
Richard
16 years of experience
BMW 328i - Brake Pads Replacement (Rear) - Dublin, California
Richard was an outstanding mechanic. After he arrived, the first thing he did after taking the initial pictures of the car is an assessment into what work was asked. He quickly verified that I did not need brake fluid and my front brakes even-though it affects his personal commitments and time, which I think is highly appreciate and thorough professional. He was detailed oriented - took time to make sure that all the issue are fixed. He paid attention to detail in overall car assessment- he goes above and beyond. He was also teaching me through the process which I found highly educational and appreciate him providing the information ensuring and making me comfortable what he was doing and explaining patiently all the questions that was thrown at him. When he works - you know he's highly skilled and experienced and knows his stuff. He is highly experienced. He is a "Master" mechanic in true sense :)

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