Transient P1133 Failures (Tech 2 History) After SLP Lid Install & Trim Reset
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TECH Regular
Joined: Aug 2009
Posts: 432
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From: Grand Valley
Transient P1133 Failures (Tech 2 History) After SLP Lid Install & Trim Reset Hey guys,
Looking for some insight from anyone who has chased transient O_2 switching speed monitors on legacy GM Gen III PCMs (specifically a 2002 LS1 F-Body operating system).
Background: The car has never had a pending or confirmed code stored in its life. I recently fixed a minor vacuum leak connection at the AIR pump solenoid, installed an aftermarket SLP air lid with a smooth bellows/cold air pack, and performed a full fuel trim reset.
Since the reset, I’ve noticed two isolated "Fail" ticks on the Tech 2 history scoreboard for P1133 (Bank 1, Sensor 1). However, the overall counter has 14 clean passes, and it has still never tripped a pending code, a confirmed code, or illuminated the MIL on the dash. I know it's a Type B code requiring consecutive trip failures, so the software is filtering it out, but I’m trying to nail down the exact root cause of the micro-events.
The Two Isolated Failures:
1 Fail #1: Occurred early on during the initial learning drive cycle right after the trim reset. Ambient temperature outside was cold, right around 3°C.
2 Fail #2: Occurred the other day following a warm restart. I sat for a few minutes, fired it up, and immediately drove right out onto a high-speed highway, followed shortly after by a long coast down a big hill.
The Data Logs:
When I dig into the actual scanner data, the execution parameters and raw sensor operation look immaculate.
Voltages & Range: Both front sensors have fantastic amplitude, hitting full rich (near 1.0\text{V}) and sweeping clean down to full lean (0\text{V}). There is zero lazy scaling or chemical contamination.
Switching Frequency: Warmed up in Closed Loop, they are tracking beautifully right together, averaging a crisp 31\text{--}33 switches per minute.
Fuel Trims: Intentionally tight and well-balanced across both banks. My stabilized Long-Term Fuel Trims (LTFT) are locked in solid, hovering cleanly around 7.8\%\text{--}9.4\% at standard highway cruise and tracking tightly between 6.1\%\text{--}7.3\% at idle. Total fuel trims are averaging perfectly right around 8.5\% on Bank 1 and 7.9\% on Bank 2.
My Theory / Question for the Forum:
Since both front sensors are identical, brand-new denso units, I'm leaning toward this being a purely environmental/thermodynamic byproduct rather than actual hardware degradation.
My thinking is that the SLP lid pack is flowing a significantly denser, higher-velocity, colder fresh air charge into the cylinders at low-load cruise. On a cold 3°C day—or immediately following a warm restart before the physical under-car steel is completely heat-soaked—this increased cold airflow drops the baseline exhaust gas temperature just enough to chill the sensor element.
Furthermore, I suspect that long coast down the hill right after hitting the highway played a major role in Fail #2. Dropping into a prolonged coast right at that transition cut the thermal input to the exhaust drastically while cold air was rammed down the pipes and under the car. When the PCM monitors its timing windows, Bank 1 misses the lookup table's switch target by a fraction of a second, ticks a single fail, and then spends the rest of the day racking up perfect passes once fully hot.
Have any of you calibrators seen the SLP lid/cold air packs consistently shift the thermal blueprint enough to trigger transient P1133 history notches on light highway transitions or extended coasts? Should I expect this to occasionally pop up as an environmental fluke under these conditions
Looking for some insight from anyone who has chased transient O_2 switching speed monitors on legacy GM Gen III PCMs (specifically a 2002 LS1 F-Body operating system).
Background: The car has never had a pending or confirmed code stored in its life. I recently fixed a minor vacuum leak connection at the AIR pump solenoid, installed an aftermarket SLP air lid with a smooth bellows/cold air pack, and performed a full fuel trim reset.
Since the reset, I’ve noticed two isolated "Fail" ticks on the Tech 2 history scoreboard for P1133 (Bank 1, Sensor 1). However, the overall counter has 14 clean passes, and it has still never tripped a pending code, a confirmed code, or illuminated the MIL on the dash. I know it's a Type B code requiring consecutive trip failures, so the software is filtering it out, but I’m trying to nail down the exact root cause of the micro-events.
The Two Isolated Failures:
1 Fail #1: Occurred early on during the initial learning drive cycle right after the trim reset. Ambient temperature outside was cold, right around 3°C.
2 Fail #2: Occurred the other day following a warm restart. I sat for a few minutes, fired it up, and immediately drove right out onto a high-speed highway, followed shortly after by a long coast down a big hill.
The Data Logs:
When I dig into the actual scanner data, the execution parameters and raw sensor operation look immaculate.
Voltages & Range: Both front sensors have fantastic amplitude, hitting full rich (near 1.0\text{V}) and sweeping clean down to full lean (0\text{V}). There is zero lazy scaling or chemical contamination.
Switching Frequency: Warmed up in Closed Loop, they are tracking beautifully right together, averaging a crisp 31\text{--}33 switches per minute.
Fuel Trims: Intentionally tight and well-balanced across both banks. My stabilized Long-Term Fuel Trims (LTFT) are locked in solid, hovering cleanly around 7.8\%\text{--}9.4\% at standard highway cruise and tracking tightly between 6.1\%\text{--}7.3\% at idle. Total fuel trims are averaging perfectly right around 8.5\% on Bank 1 and 7.9\% on Bank 2.
My Theory / Question for the Forum:
Since both front sensors are identical, brand-new denso units, I'm leaning toward this being a purely environmental/thermodynamic byproduct rather than actual hardware degradation.
My thinking is that the SLP lid pack is flowing a significantly denser, higher-velocity, colder fresh air charge into the cylinders at low-load cruise. On a cold 3°C day—or immediately following a warm restart before the physical under-car steel is completely heat-soaked—this increased cold airflow drops the baseline exhaust gas temperature just enough to chill the sensor element.
Furthermore, I suspect that long coast down the hill right after hitting the highway played a major role in Fail #2. Dropping into a prolonged coast right at that transition cut the thermal input to the exhaust drastically while cold air was rammed down the pipes and under the car. When the PCM monitors its timing windows, Bank 1 misses the lookup table's switch target by a fraction of a second, ticks a single fail, and then spends the rest of the day racking up perfect passes once fully hot.
Have any of you calibrators seen the SLP lid/cold air packs consistently shift the thermal blueprint enough to trigger transient P1133 history notches on light highway transitions or extended coasts? Should I expect this to occasionally pop up as an environmental fluke under these conditions
Last edited by Ape Man; May 19, 2026 at 05:38 AM.
The problem is you have new Denso O2s that switch just differently enough than what the PCM was originally calibrated for. I don't believe there are any tables that are readily accessible to alter the behavior of the diag test for insufficient switching, and integral delay is just for control not for diag. Generally speaking, just turn off the codes.
Thread Starter
TECH Regular
Joined: Aug 2009
Posts: 432
Likes: 77
From: Grand Valley
The problem is you have new Denso O2s that switch just differently enough than what the PCM was originally calibrated for. I don't believe there are any tables that are readily accessible to alter the behavior of the diag test for insufficient switching, and integral delay is just for control not for diag. Generally speaking, just turn off the codes.
