You plug the scan tool into the port under the dash, turn the key on, and the screen says it cannot connect. No VIN. No modules. No codes. Just a tool that will not talk to the car.<\/p>\n
That is not the same problem as a U0100 code. U0100 means other modules lost contact with the ECM or PCM on the vehicle network. Here, the scan tool itself never got a conversation started. The break is between the tester and the computer, not between two computers already running on the car.<\/p>\n
This blog covers why a scan tool has no communication with the ECU, what to check at the DLC first, and when a dead module is actually the cause.<\/p>\n
Scan tool no communication means the tester cannot open a data link with the engine or powertrain computer. On most late-model cars that link is CAN, using pins 6 and 14 in the OBD-II port, plus battery power on pin 16 and grounds on pins 4 and 5.<\/p>\n
If any of those circuits are dead, the tool never sees the ECU. A failed module can cause the same blank screen, but so can a blown DLC fuse, a key that is not in the run position, or a shorted CAN line. Confirm the port and ignition first. Then decide whether the computer is even powered.<\/p>\n
These are the usual breaks in the path from the scanner to the ECU, starting with the ones you can check at the port.<\/p>\n
Pin 16 should have battery voltage with the key off. If it doesn’t, the scan tool may not even power up, or it may run off its own battery and still fail to talk to the car. A blown cigarette-lighter or OBD fuse is a common reason the port is dead while the rest of the car still cranks.<\/p>\n
Check pin 16 to ground with a meter before you blame the ECU. If the fuse is open, find out why it blew instead of stacking another fuse on top of a short.<\/p>\n
Pins 4 and 5 are the signal and chassis grounds for the diagnostic connector. A loose or corroded ground can leave the tool powered but unable to hold a clean conversation with the module. The screen may freeze, drop, or report no communication even though the ECU is awake.<\/p>\n
Clean the grounds at the port and at the nearby body points. Then retest with a known-good scan tool so you are not chasing a tester fault.<\/p>\n
On CAN vehicles, pin 6 is CAN-H and pin 14 is CAN-L. A short, open, or aftermarket alarm spliced into those wires can keep every scan tool from connecting. Resistance across 6 and 14 with the battery disconnected is usually close to 60 ohms when both terminating resistors are intact.<\/p>\n
This is also where a U0100 code<\/a> can show up on a second module if the network is damaged but some controllers are still alive. Tool-to-ECU silence and module-to-module loss can share a wiring cause, but they are not the same diagnosis.<\/p>\n Many modules only talk with the key in run. If the ignition switch is worn, or the ECU fuse and main relay never close, the computer is asleep and the scan tool has nothing to address. Confirm the key position, then check the ECU or PCM fuses and the power relay circuit.<\/p>\n A related power-relay fault is P0685<\/a>, but you will not see that code until something on the network can talk. If the module has no power, the scanner stays blank.<\/p>\n When the DLC has power and ground, CAN is intact, and the module still has battery and ignition at its connector, a silent computer is on the table. Internal damage can keep it from answering anyone, including a professional scan tool.<\/p>\n That is a different picture from a stored P0606 code<\/a>, which at least means the module was alive enough to set a fault. For location and access on Chrysler products, see Dodge ECM location<\/a> before you pull the unit.<\/p>\n The port tells you more than the symptom does. Measure before you order a module.<\/p>\n4. Ignition Off or No ECU Power<\/h3>\n
5. A Failed ECU or PCM<\/h3>\n
How to Tell These Causes Apart<\/h2>\n