Okay....so here's what's up. 1999 Prelude SH. Purchased used about a month or so ago with 115k miles. When purchased the car was running poorly (super rich, horrible mileage, bouncing & high idle) on a P13 ECU. I bought and had chipped (A01 mobilizer from Xenocron) a proper P5P ECU. The car is/was modified, but primarily with bolt ons....S2 I/M and throttle body....AEM CAI.....severely lowered on Twin coilovers.....cat back exhaust system (both O2s and cat are present). Oh, and I should mention the CEL was also already on, but I neglected to check the OBD1 codes prior to Installing the P5P. So the P5P goes in, idle isn't AS HIGH, but still high, like 2000 rpms high. I checked the OBD2 codes, and they are P0108, 0113, 0118 & 0122....all high voltage detected in the ECT, IAT, MAP and TPS. I'm thinking this would indicate a ground short on that harness, but where? Also, I checked the voltage going TO the sensors, IAT 5 volts, MAP 5 volts, but the ECT is getting 12 volts, which confused me. To verify it's not an ECU issue, I did plug the p14 back in and checked the blink codes...they were the same, 06, 07, & 10.….. ECT, IAT, and MAP. Also, crawled up underneath and found one of the sensors, I think it's another ECT down by the oil filter, had it's wired broken off, and just hanging. Thought that might be the problem (haven't fixed yet, but) jumped those two wires and had not change in the live data.......ECT and IAT were absolutely STUCK at -40. So......does anyone have any clues or hints to try and isolate WHERE the short could be? Or, is there another possibility? I SERIOUSLY APPRECIATE ANY help guys (and gals).....Thanks.

 

You have a break in the sensor ground between the ECU and the referenced sensors on the engine harness. I believe there is a junction block under the intake manifold that joins the incoming sensor ground from the ECU to all sensor ground points for the ECT, IAT, EGR, and TPS. The interesting part would be the MAP sensor in your equation... it should have a dedicated ground from the ECU. Now, the OBD-1 codes that you referenced do NOT include a MAP code... instead, you have a TPS (7) code. The OBD-1 code for MAP is 3.

I would check continuity between the sensor ground at the ECU and where it meets the engine harness... and then from that junction plug to each sensor. Wherever you fail to have continuity... look deeper at the blocks.