Sorry to hear about your troubles. Hopefully they didn't break off the plastic port on the vacuum canister and therefore the reason why they didn't hook it up.

PO135 should be able to be dealt with fairly easily if they follow the diagnostic charts. Probably something related to the last engine install if code wasn't setting before. I am thinking they should deal with it gratis. Maybe a disconnected or damaged O2 sensor heater wire. If you end up wanting to diagnose yourself, let us know and we will help. Don't keep throwing parts at it. You do know there are three oxygen sensors in your 2001 Blazer, right????

FYI below is some factory info on the PO135 and it will not set in the presence of a PO300. So it makes sense that you are now getting it if the PO300 is no longer setting. PO135 can only set during initial warmup of the vehicle and only once per ignition cycle. So driving it for an extended time or restarting while warmed up will not evaluate if it is repaired or not. Only one test is done per engine warmup.

Good luck and best wishes!

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P0135 CIRCUIT DESCRIPTION
The Heated Oxygen Sensor (HO2S) is a sensor designed to create a voltage relative to the oxygen content in the engine exhaust stream. The control module supplies the HO2S with signal high and low circuits. Ignition voltage and ground are supplied to the HO2S heater by independent circuits. The oxygen content of the exhaust indicates when the engine is operating lean or rich. When the HO2S detects that the engine is operating rich, the signal voltage is high, and decreases the signal voltage as the engine runs leaner. This oscillation above and below the bias voltage, sometimes referred to as activity or switching, can be monitored with the HO2S signal voltage.

The HO2S contains a heater that is necessary in order to quickly warm the sensor to operating temperature. The heater also maintains the operating temperature during extended idle conditions. The HO2S needs to be at a high temperature in order to produce a voltage. When the HO2S reaches operating temperature, the control module monitors the HO2S bias, or reference, voltage. The control module the HO2S signal voltage for Closed Loop fuel control. During normal Closed Loop fuel control operation, the control module will add fuel, or enrich the mixture, when the HO2S detects a lean exhaust content. The control module will subtract fuel, or "lean-out" the mixture, when the HO2S detects a rich exhaust condition.

This Diagnostic Trouble Code (DTC) determines if the HO2S heater circuit is functioning properly by monitoring the time required for the HO2S to reach the operating temperature. This DTC sets when the Powertrain Control Module (PCM) fails to detect HO2S voltage transitions above and below the bias range within a specified amount of time.

CONDITIONS FOR RUNNING THE DTC

• DTCs P0101, P0102, P0103, P0106, P0107, P0108, P0112, P0113, P0117, P0118, P0121, P0122, P0123, P0300, P0351, P0401, P0404, P0405, P0440, P0442, P0446, P0452, P0453, P1258, P1404, or P1441 are not set.
• The HO2S voltage is between 425-75 mV at engine startup.
• The Intake Air Temperature (IAT) and the Engine Coolant Temperature (ECT) are less than 50°C (122°F) , and are within 8°C (14.5°F) of each other at engine start-up.
• The ignition 1 signal is between 9-18 volts.
• Intrusive tests are not in progress.
• The scan tool output controls are not active.
• The scan tool is disconnected.

CONDITIONS FOR SETTING THE DTC
The HO2S voltage remains within 150 mV of the startup voltage for a predetermined amount of time,

ACTION TAKEN WHEN THE DTC SETS

• The control module illuminates the Malfunction Indicator Lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.
• The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the Failure Records.

CONDITIONS FOR CLEARING THE MIL/DTC

• The control module turns OFF the Malfunction Indicator Lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
• A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
• A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
• Use a scan tool in order to clear the MIL and the DTC.

DIAGNOSTIC AIDS

IMPORTANT: Remove any debris from the PCM connector surfaces before servicing the PCM. Inspect the PCM connector gaskets when diagnosing or replacing the PCM. Ensure that the gaskets are installed correctly. The gaskets prevent water intrusion into the PCM.

• Use the Freeze Frame/Failure Records in order to help locate an intermittent condition. If you cannot duplicate the DTC, the information in the Freeze Frame/Failure Records can help to determine how many miles have occurred since the DTC set. The Fail Counter and the Pass Counter can also help determine how many ignition cycles the diagnostic reported a pass or a fail. In order to isolate when the DTC failed, operate the vehicle within the same Freeze Frame conditions such as RPM, load, vehicle speed, temperature, etc., that you observed. For an intermittent condition, refer to Symptoms - Computers and Control Systems. See: Powertrain Management\Computers and Control Systems\Testing and Inspection\Symptom Related Diagnostic Procedures These conditions include:
• The RPM
• The vehicle load
• The vehicle speed
• The temperature
• The heater diagnostic will only run on a cold start and run once per ignition cycle.
• An oxygen supply inside the HO2S is necessary for proper operation. The HO2S wires provide the supply of oxygen. Inspect the HO2S wires and inspect the connections for breaks or for contamination Refer to Heated Oxygen Sensor (HO2S) Wiring Repairs in Diagrams.