My G35 has an issue where if it rains too hard water gets into the relay box over by where the battery is. If this happens and I try to start the car it goes into limp home mode with the CEL on. If I let it dry out it will run just fine and all I have to do is clear the CEL.

This happened recently but this time I ignored the CEL and the light turned off by itself except there are still error codes in the ECU. When I try to clear the codes using the accelerator pedal procedure the codes are still there. If I disconnect the battery and let it sit the codes remain. What is going on? Why won't the codes clear?

The codes are:
U1000 CAN communication circuit
P1815 Manual mode switch
P1705 Throttle position sensor
C1136 Engine signal 6

 

Looks like a failed TPS, it's part of the throttle body. Replace it with a Hitachi, they were the OEM manufacturer.

You really need to fix that cowl drain issue so your IPDM doesn't get soaked.

U1000 means one of the system computers isn't communicating, sort of hard to nail this one down without the dealership Consult-II/III tool.

C1136 is probably because of the missing throttle position data.

Ignore the manumatic switch code for now.

 

My question is why won't the codes clear even when I disconnect the battery. If the errors are reccuring then why doesn't the CEL come back on? I probably didn't explain it right. After I reset the ECU and start the car the CEL does not come back on but the errors are still in the ECU when read by a code reader.

 

Every time you power the car the computer network does a quick self-check to make sure everything is there. If one of the computers isn't communicating it's going to throw that U1000 code because the car knows something is missing.

Battery disconnect will clear codes, but you need to pump the brake pedal to discharge ALL the stored power (turning on the brake lights does this) and then wait about 10 minutes. The problem is that code is probably coming back immediately when you reconnect the battery and it does it's self-check.

The CEL light doesn't ALWAYS turn on, it turns on for anything that's important enough that you need to either:

A. Schedule service at your earliest convenience, this will be a solid check engine light.

B. Pull the car over IMMEDIATELY and have the vehicle towed to a service shop, this will be a blinking check engine light.

Additionally there are a lot of "hidden" codes that don't turn on the CEL, these are designed so the dealership can monitor things that may or may not be a problem in the future when the vehicle comes in for regular scheduled service visits. It's called a "two trip notification", if the error only happens a single time it stores the code, usually until the engine has been turned on and off a certain number of times then it will automatically clear the code (I think most Nissan cars are 15 cycles to clear). Some codes though will not self-clear so the dealership can always catch them on the next service visit. Things like cam/crank/throttle position/maf, or basically all of your powertrain management codes will stay stored. Other codes like ECM backup voltage, cruise control switch, etc will self-clear eventually because when you do something like disconnect the battery you don't want to have a backup power code stored indefinitely.

Almost every code will turn on the CEL though if it actually has two consecutive trips, the ECM knows it's supposed to be receiving data from everything at a certain rate, an occasional dropped signal it can deal with, two consecutive dropped signals definitely means there's a problem and it will trigger the CEL either solid or flashing depending on severity.

By far the BEST way to clear codes though is to command the ECM to clear using an OBD2 scanner, the battery method does work but it's FAR easier to just plug into the diagnostic port and push a button.

 

There are a LOT of computers on these cars, they can all send their own specific codes, they're all connected together using a network system called CANBUS. Doesn't require a router, it's more like the old parallel computer networking systems, they all just plug directly into one another.

Each of them receives a certain set of data, processes and outputs to command whatever system it's running. They also SHARE a lot of the same data.

ECM - Handles engine control, is also the main computer for communications. Also outputs a lot of shared data onto CANBUS like steering angle, throttle position, etc which is used by other computers.

TCM - Handles transmission shifting, requires a lot of other data from other computers.

Combination meter (instrument cluster) - Obviously displays a lot of data that it gets via CANBUS but also sends out data like the manumatic shift up/down signal from your shifter, this tells the TCM to manually shift up/down.

AC Auto Amp - V35 and V36 chassis cars use this for different stuff, V36 the fuel gauge info goes through this computer first, V35 it's mostly just for controlling the air conditioner which is a pretty massive system with a LOT of sensors and outputs.

BCM - This not only handles window, locks, and seat controls but is also the main computer for NATS/IVIS (Nissan Anti Theft System / Infiniti Vehicle Immobilizing System), your key has a transponder in it that the BCM can see when you're within range. As long as it recognizes the fob it will allow for normal vehicle function, if not it will do things like trigger the alarm, or lock out the ability to even crank the engine.

iKEY - Has it's own separate computer to interface the BCM.

IPDM - Intelligent Power Distribution Module, this is the engine bay fuse box, it reads signals from the BCM and ECM to handle functions like turning on the headlight relay, starter relay, etc.

ABS/VDC/TCS - Antilock brakes, vehicle dynamic control, and traction control system. This computer monitors wheel speed on all four corners, if it detects something slipping it will CANBUS command the ECM to close throttle (your gas pedal is just a switch, no cable), vehicle dynamic control and traction control use the steering angle sensor and the yaw sensor (fancy digital compass mounted in the center console) to determine if the car is actually going the direction you're pointing the steering wheel. This computer can actuate the brakes without you touching the pedal, there is a pump motor to build brake pressure then it can open/close solenoids on the ABS valve block to apply braking pressure to each of the four corners. When you first drive the car after turning it on, when you hit 5MPH the system will do a self-test, you can hear a faint "whirring and grinding" sound for 1 second if you listen closely, that motor is right in front of the brake booster.

RAS/HICAS - Rear active steering / High Capacity Actively Controlled Steering. For the V35 chassis this was only available on 2006-2007 coupes with 6MT and it was optional, this was also available on several V36 cars. There is a motor controlled power steeering rack on the rear axle that can be actuated to turn the rear tires, this system is only for higher speed, helps maintain speed coming out of a corner.

SRS - Supplemental Restraint System, this monitors seatbelt switches, seat cushion pressure sensors, crash zone sensors, and will deploy whichever airbags need to be deployed during a crash, also pops the seat belt auto retractors.

Driver Seat Control Unit - For some reason the seats have their own computer. Probably to save on wiring so all the switches don't have to have their own wires out to motors. Switches can just talk to the BCM which then sends the data signal via the two-wire CANBUS network out to the actual seat controller.

Hope this helps shed a little light on how the system actually works.

 

Thanks for the info.