← Back to list

P1742 and P0743 Fault Diagnosis: Mazda 626 (CD4E) Automatic Transmission with TCC Solenoid Issues

Model: Mazda 626 Fault Code: P0743 Posted: 2007-05-15 22:43

I'm seeking help diagnosing persistent transmission issues on my 1999 Mazda 626 (4-cylinder, CD4E automatic transmission). After installing a remanufactured transmission, I began experiencing the O/D light flashing and two diagnostic trouble codes: P1742 (TCC solenoid electrical short) and P0743 (TCC circuit electrical failure). The original transmission remanufacturer failed to respond to my complaint. I took the vehicle to a specialized transmission technician who identified a defective TCC solenoid pack. I replaced it with a new unit that meets specifications (12.5–14 ohms resistance), as the old one measured only 2 ohms. Despite this replacement, both P1742 and P0743 codes persist. The remanufacturer claims the issue lies in the vehicle's control module rather than the transmission itself. To investigate further, I disconnected the TCC solenoid signal wire from the Powertrain Control Module (PCM) and connected it to a 14 ohm test lamp during a drive cycle. The PCM should have provided a ground signal, but no response was observed. My key questions are: - Could a shorted TCC solenoid have damaged or disabled the PCM? - What type of signal does the PCM send to the TCC solenoid—solid ground or a modulated signal? - Is there any known electrical protection in the PCM against such faults? I am currently unable to afford professional diagnostics and have limited access to tools. I need clear, factual guidance on next steps. Additional symptoms include: - Uneven shift firmness (smooth from 1st to 2nd, rough from 2nd to 3rd, smooth from 3rd to 4th) - Failure to downshift during deceleration; engine attempts to accelerate at high RPMs when restarting - Engine stalling upon stoppage (RPM drops below 500), followed by sudden recovery after warming up I suspect the issue may stem from internal transmission components (e.g., valve body, clutches), but the persistent codes suggest a potential controller problem. I am considering whether to replace the PCM or pursue further diagnostics. What should I check next? Is there a known range sensor failure pattern in Mazda 626 CD4E models? Can a faulty range sensor cause these symptoms and trigger P1742/P0743? Any technical references, wiring diagrams, or signal specifications for the TCC solenoid control signal would be greatly appreciated.

Related fault codes
P0743P1742
Comments (12)
Anonymous 2007-05-16 07:20

Have you checked the range sensor? It's a common failure point in Mazda and Ford vehicles with automatic transmissions.

Anonymous 2007-05-16 07:48

The transmission specialist performed a 15-point inspection but didn't explain what each check means. I’ve since found online resources on how to test the range sensor—this will be my next step. Thank you, Attila.

Anonymous 2007-05-16 11:09

A 15-point inspection sounds like a vague and unhelpful service.

Anonymous 2007-05-16 13:44

I was very dissatisfied with the transmission shop. I paid $190 for an assessment, but they couldn't determine whether the issue is in the transmission or control module. Without this clarity, the remanufacturer won’t fix it. My symptoms include: - Uneven shift firmness (smooth 1st to 2nd, rough 2nd to 3rd, smooth 3rd to 4th) - No downshift during deceleration—engine tries to accelerate from 1000 RPM when restarting - Engine stalls upon stopping, RPM drops below 500, then recovers abruptly after warming up - O/D light flashes (P1742 and P0743 codes) I believe the issue is likely internal in the transmission (valve body or clutches), but the remanufacturer claims it's a vehicle-level problem. I suspect that a shorted solenoid may have caused electrical damage to the PCM. Does the PCM have overvoltage protection? If not, could this explain persistent codes?

Anonymous 2007-05-16 14:12

The control module does influence shift firmness based on inputs from sensors and valve body. A misadjusted shifter cable or linkage can also cause these symptoms. Who performed the transmission overhaul? I recommend visiting a more experienced shop to check the range sensor, cable adjustment, or trying a part swap—start with the range sensor. Be prepared for labor costs; reputable shops may charge several hours. A Mazda dealer might be an option, though they often avoid providing technical details over the phone.

Anonymous 2007-05-21 20:59

I confirmed the range sensor is open between terminals 1 and 7 during all gear shifts—effectively dead. I will now replace it with a new unit to test functionality.

Anonymous 2007-05-22 07:47

This is indeed a common failure point in these models.

Anonymous 2007-05-22 09:42

I discovered that the wiring diagram in my repair manual incorrectly mirrored the actual connector pinout—the terminal numbering was reversed. After verifying with correct pins, the range sensor appears functional. This is disappointing. I tried contacting a Mazda dealership, but they are unhelpful—only willing to perform work without providing explanations. I asked about the PCM’s signal to the TCC solenoid (solid ground or modulated), and was told it's a 'safety' issue with no further details. I don’t understand why this would be considered safety-related; I suspect it's more about profit. What should I check next? Should I replace the control module? Where can I find reliable technical documentation on TCC solenoid signal characteristics?

Anonymous 2007-05-22 09:47

Before replacing the controller, try verifying the range sensor or checking shifter cable adjustment. In my experience, transmission control module failures are rare—most issues stem from sensors or mechanical components.

Anonymous 2007-05-22 16:44

Who supplied the remanufactured transmission? Was it a full rebuild or an overhaul of the original unit? Did they include a new torque converter and perform re&re (removal, inspection, replacement) procedures?

Anonymous 2007-05-27 06:37

I'm experiencing the same issue: 1999 Mazda 626 with new transmission, O/D light flashes, engine light on. Diagnostics list it as a slipping transmission—but the car runs fine otherwise except for occasional hard shifts.

Anonymous 2007-05-28 08:17

Did you install a new range sensor with the remanufactured unit? Was the shifter cable properly installed and adjusted?