QuestionHello all knowledgeable Experts,
I recently had my speed sensor and timing belt replaced 3 weeks ago by my local mechanic. About 7 days ago my check engine light came on and the codes read: U3600 and P0720. My mechanic said that the computer in my 2001 Hyundai Elantra is not reading the new speed sensor they just installed, and that i should "Drive the car till it dies". My gut instinct says this isn't right, but i know nothing about these codes. I need advice on what to do.
Please Help.
-Thank You
AnswerP0720 indicates a problem with the output speed sensor signal. U3600 indicates a network communication issue, a problem that's impossible for this vehicle to have since it doesn't have a communication network. It's most likely indicative of a communication issue between the scan tool/code reader and the vehicle.
There are at least four speed sensors on this vehicle-- an input speed sensor and an output speed sensor for the transmission control module (TCM) to determine input and output shaft rotation speeds, a vehicle speed sensor for the TCM and instrument cluster, and the right front wheel speed sensor, which the engine control module (ECM) uses to determine engine speed.
So, the first step is to determine whether the correct sensor was replaced. The output speed sensor is on the left side rear of the top of the transmission, almost directly under the brake master cylinder. A new sensor should appear clean in comparison to its surroundings.
The TCM infers problems with the output speed sensor by comparing its readings with those that would be expected based on the input speed sensor and vehicle speed sensor readings. So, if output speed sensor replacement didn't resolve the issue, it's important to check the plausibility of the readings from these sensors as well as performing at least a cursory visual inspection of the wiring for the sensor.
If you're being told the car isn't worth fixing, a good explanation for this should follow. For example, the problem could be in the transmission itself, requiring a transmission replacement or overhaul to resolve. In this case, the age, mileage, and condition of the vehicle may not justify such an expense. On the other hand, if they're telling you they don't know what the problem is, then I think they're just dodging the responsibility of diagnosing the problem accurately. In essence, they're telling you in this case, "We can't figure out your problem, so just keep driving until you can't." You may need to find a more capable shop to get this issue repaired.
