General Motors has been making automatic transmissions since the 1940s, and they’ve developed a number of proprietary ways to monitor how their gearboxes work. One of them is the P1870 diagnostic trouble code, which gets triggered by a number of performance related parameters. Depending on the GM transmission, the P1870 DTC can be set by everything from dirty transmission fluid to a torque converter issue. But no matter what the root cause, you can certainly expect drivability issues.
How Serious is the Code?
The GM transmission fault code P1870 is particularly serious, because it typically indicates functionality problem. When the vehicle exceeds acceptable RPM parameters before a shift occurs, this transmission DTC will be set. Maximum line pressure will be commanded and TCC apply/torque converter clutch apply will be disabled. This happens to prevent damage to other transmission components. However the increased hydraulic pressure will often damage other transmission parts like seals, bearings clutches and bands.
Symptoms of a P1870 include:
- Delayed shifts
- Harsh shifts
- Lack of fourth gear/overdrive
- Increased engine RPMs
- Poor performance
- Low transmission fluid
- Dirty transmission fluid
- Wrong damper plate used
- Wrong TCC clutch material
- Worn isolator valve bore
- Worn TCC regulator valve bore
- Worn actuator feed limit valve
How to Diagnose the Code?
When diagnosing a P1870 diagnostic trouble code, OBDII diagnostic trouble code data is extremely important. However, extensive mechanical experience may be needed to work through all the issues that could be related to this common transmission problem.
Common Mistakes When Diagnosing
It’s not uncommon when diagnosing a P1870 DTC, to automatically assume that the transmission is at fault. Many times, can the engine misfire related to a coil, oxygen sensor or mass airflow sensor issues.
What Repairs Will Fix P1870?
- Change the transmission fluid//flush the transmission
- Install a high quality in-line transmission filter
- Repair worn transmission valve bores
- Replace TCC clutch
- Reprogram ECU
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